I have not yet seen a place where the limitations put by the game on 2B mapping (mapping that requires the player to use 2 keys to play the map). So I created this thread for it.
Feel free to link to other places where information on 2B limitations can be found, share your own knowledge or even double check on already posted answers that might be outdated or incorrect.
Please do note the date of new info found like so (day/month/year) if you ever edit your post. That way people will have an easier time knowing if it needs a doublecheck
Here's what I found right now:
Note: Everything I found unless specified otherwise or I messed up (it happens sometimes) was found on a map with those settings:
BPM=164
CS=3
AR=4
OD=0
HP=6
Slider tick rate=1
[SUMMARY OF WHAT'S IN HERE IN ORDER]
-General
-Autofail
-Circles
-Circles with sliders
-Sliders
-Sliderball priority
-Sliders with spinners
-HP
-Funny stuff I found along the way
[GENERAL]
-(03/04/2021) The K1 and M1 inputs as well as the K2 and M2 inputs cannot be pressed at the same time or else they will be ignored by the game (note: the game won't consider the additional inputs as being actual clicking but the counter will behave weirdly. If the key press is given before the mouse press, the mouse input will be ignored by the counter unless you release the key before the mouse button but if the mouse press is given first, it will appear to be replaced by the key press as if you let go of the mouse button to hit the key and if you let go of the key before the mouse button, the counter will take into account a new mouse press). Therefore, at most, 2 inputs can be given at the same time.
[AUTOFAIL]
-(03/04/21) Autofail is an issue caused by certain 2B techniques that will prevent scores on a map from uploading. For more info on it, you can watch the pishifat video linked under the post as a reply by TheSpook39. It is for the most part still relevant and I won't reestablish the elements in there that still apply.
-(03/04/21) The current indicator of a map being subject to autofail seems to be the message
saying "An error has occured during the computation of the score" (might not be exactly that because I had to translate it from french to english) popping up at the bottom right of the screen at the end of the map. (I didn't verify this but it makes a lot of sense to have a message saying "I can't calculate what amount of pp you should get" pop up when the game indeed can't calculate it and so, can't put the score online)
For context, I noticed that a slider with circles overlapped on its slidertail stopped working halfway like in the picture below and I started experimenting
-(18/01/2021) Found a weird thing. I had the whole map basically in the exact same situation as when the yellow slider right over this stopped working and I put a single circle after the slider and the slider stopped working later.
I moved the circle around in the timeline including during the slider's duration and the slider always stopped working there. Not sure to understand what's going on here.
-(27/01/2021) I kept moving the 4th circle around until I had the idea to place it before the 3rd one (so now there are just 3 circles instead of 2 before the last one).
Doing this made the slider stop working after the version with only 3 circles but before the version where the 4th circle was placed later.
[CIRCLES]
-(18/01/2021) Two circles can be clicked at the same time.
[CIRCLES WITH SLIDERS]
-(18/01/2021) Circles can be clicked during a slider's duration.
The slider will however stop working and won't be giving any points at the end.
Also, the score won't compute properly. (cf.[AUTOFAIL])
[SLIDERS]
-(29/11/2020) Two sliderballs can't be active (considered as clicked by the game) at the same time.
(For those curious, the green slider gives only a 100 because the ball was inactive when it passed over the tick)
-(01/12/2020) If 2 sliders of equal duration are placed on the same tick and the slider heads overlap, double tapping the overlap will cause a complete miss of the slider placed second but the one placed first will be playable normally.
-(01/12/2020) If 2 sliders of equal duration are placed on the same tick and part of the slider heads don't overlap, clicking the non overlapped parts of each sliderhead one after the other will allow each head to be clicked but prevent the sliderballs from appearing therefore forcing the player to get at best two 50s.
-(01/12/2020) If two sliders of different duration are placed on the same tick then clicking the non overlapped parts alternatively will activate the ball of the slider placed first (cf. [slider ball priority] to see why I think it works that way) and then respect the rule saying that 2 sliderballs can't be active at the same time.
[SLIDERBALL PRIORITY]
-(01/12/2020) Sliderball priority is determined by which slider is first in the .osu file which means which is placed first (note: cut+paste did count as placing in a different order for equal duration sliders but I had to manually change the position of the sliders in the file for different duration sliders). Here are the experiments that led me to this conclusion:
DIFFERENT DURATION:
-Red slider placed first, Yellow slider placed second
Red has priority
-Yellow slider placed first, Red slider placed second
Yellow has priority
EQUAL DURATION:
-Blue slider placed first, Green slider placed second
Blue has priority
-Green slider placed first, Blue slider placed second
Green has priority
[SLIDERS WITH SPINNERS]
Similar rules seem to apply to sliders and spinners:
-(01/12/2020) If a slider is hit while a spinner is on screen, it's sliderball won't appear but will reappear as soon as the spinner is done
[HP]
-(18/01/2021) Sliders of unequal duration put on the same beat will absolutely destroy the player's HP at the end of the first slider. Here's a an example: this innocent player is playing this 2B map, hits these sliders and follows the sliderball. All is well, they have HP and they have combo
But then the first slider ends, they get a 300 and +1 combo but suddenly about the third of the HP bar is gone and their combo went away on a perfectly hit slider.
-(03/02/2021) You can even just snap them out of existence by increasing slider tick rate. Here's what happens with slider tick rate=4:
Kalm
Ded (don't mind the difference in combo, sometimes I have to do multiple playtests to get the right picture but the situation was basically the same)
[FUNNY STUFF I FOUND ALONG THE WAY]
-(18/01/2021) Funny thing: Since you can't give more than 2 key inputs at the same time this 2B pattern needs to be cheesed by clicking all 4 key inputs alternatively, doubletappind twice or streaming so fast that you stay in the OD limitation.
Feel free to link to other places where information on 2B limitations can be found, share your own knowledge or even double check on already posted answers that might be outdated or incorrect.
Please do note the date of new info found like so (day/month/year) if you ever edit your post. That way people will have an easier time knowing if it needs a doublecheck
Here's what I found right now:
Note: Everything I found unless specified otherwise or I messed up (it happens sometimes) was found on a map with those settings:
BPM=164
CS=3
AR=4
OD=0
HP=6
Slider tick rate=1
[SUMMARY OF WHAT'S IN HERE IN ORDER]
-General
-Autofail
-Circles
-Circles with sliders
-Sliders
-Sliderball priority
-Sliders with spinners
-HP
-Funny stuff I found along the way
[GENERAL]
-(03/04/2021) The K1 and M1 inputs as well as the K2 and M2 inputs cannot be pressed at the same time or else they will be ignored by the game (note: the game won't consider the additional inputs as being actual clicking but the counter will behave weirdly. If the key press is given before the mouse press, the mouse input will be ignored by the counter unless you release the key before the mouse button but if the mouse press is given first, it will appear to be replaced by the key press as if you let go of the mouse button to hit the key and if you let go of the key before the mouse button, the counter will take into account a new mouse press). Therefore, at most, 2 inputs can be given at the same time.
[AUTOFAIL]
-(03/04/21) Autofail is an issue caused by certain 2B techniques that will prevent scores on a map from uploading. For more info on it, you can watch the pishifat video linked under the post as a reply by TheSpook39. It is for the most part still relevant and I won't reestablish the elements in there that still apply.
-(03/04/21) The current indicator of a map being subject to autofail seems to be the message
saying "An error has occured during the computation of the score" (might not be exactly that because I had to translate it from french to english) popping up at the bottom right of the screen at the end of the map. (I didn't verify this but it makes a lot of sense to have a message saying "I can't calculate what amount of pp you should get" pop up when the game indeed can't calculate it and so, can't put the score online)
For context, I noticed that a slider with circles overlapped on its slidertail stopped working halfway like in the picture below and I started experimenting
-(18/01/2021) Found a weird thing. I had the whole map basically in the exact same situation as when the yellow slider right over this stopped working and I put a single circle after the slider and the slider stopped working later.
I moved the circle around in the timeline including during the slider's duration and the slider always stopped working there. Not sure to understand what's going on here.
-(27/01/2021) I kept moving the 4th circle around until I had the idea to place it before the 3rd one (so now there are just 3 circles instead of 2 before the last one).
Doing this made the slider stop working after the version with only 3 circles but before the version where the 4th circle was placed later.
[CIRCLES]
-(18/01/2021) Two circles can be clicked at the same time.
[CIRCLES WITH SLIDERS]
-(18/01/2021) Circles can be clicked during a slider's duration.
The slider will however stop working and won't be giving any points at the end.
Also, the score won't compute properly. (cf.[AUTOFAIL])
[SLIDERS]
-(29/11/2020) Two sliderballs can't be active (considered as clicked by the game) at the same time.
(For those curious, the green slider gives only a 100 because the ball was inactive when it passed over the tick)
-(01/12/2020) If 2 sliders of equal duration are placed on the same tick and the slider heads overlap, double tapping the overlap will cause a complete miss of the slider placed second but the one placed first will be playable normally.
-(01/12/2020) If 2 sliders of equal duration are placed on the same tick and part of the slider heads don't overlap, clicking the non overlapped parts of each sliderhead one after the other will allow each head to be clicked but prevent the sliderballs from appearing therefore forcing the player to get at best two 50s.
-(01/12/2020) If two sliders of different duration are placed on the same tick then clicking the non overlapped parts alternatively will activate the ball of the slider placed first (cf. [slider ball priority] to see why I think it works that way) and then respect the rule saying that 2 sliderballs can't be active at the same time.
[SLIDERBALL PRIORITY]
-(01/12/2020) Sliderball priority is determined by which slider is first in the .osu file which means which is placed first (note: cut+paste did count as placing in a different order for equal duration sliders but I had to manually change the position of the sliders in the file for different duration sliders). Here are the experiments that led me to this conclusion:
DIFFERENT DURATION:
-Red slider placed first, Yellow slider placed second
Red has priority
-Yellow slider placed first, Red slider placed second
Yellow has priority
EQUAL DURATION:
-Blue slider placed first, Green slider placed second
Blue has priority
-Green slider placed first, Blue slider placed second
Green has priority
[SLIDERS WITH SPINNERS]
Similar rules seem to apply to sliders and spinners:
-(01/12/2020) If a slider is hit while a spinner is on screen, it's sliderball won't appear but will reappear as soon as the spinner is done
[HP]
-(18/01/2021) Sliders of unequal duration put on the same beat will absolutely destroy the player's HP at the end of the first slider. Here's a an example: this innocent player is playing this 2B map, hits these sliders and follows the sliderball. All is well, they have HP and they have combo
But then the first slider ends, they get a 300 and +1 combo but suddenly about the third of the HP bar is gone and their combo went away on a perfectly hit slider.
-(03/02/2021) You can even just snap them out of existence by increasing slider tick rate. Here's what happens with slider tick rate=4:
Kalm
Ded (don't mind the difference in combo, sometimes I have to do multiple playtests to get the right picture but the situation was basically the same)
[FUNNY STUFF I FOUND ALONG THE WAY]
-(18/01/2021) Funny thing: Since you can't give more than 2 key inputs at the same time this 2B pattern needs to be cheesed by clicking all 4 key inputs alternatively, doubletappind twice or streaming so fast that you stay in the OD limitation.