Hi, this would be a really big change in osu!, by that i mean really BIG.
What you could do is make a whole new osu! mode called osu! standard TAG
There, pp would be seperate from osu! standard.
The Rankings for maps would be for example
Cookiezi and... or Cookiezi , WubWoofWolf and ... [Depending on the TAG, TAG2 TAG3 etc.]
But, the global rankings (PP and such) it would be just like in osu! standard, since people would play with different people all the time.
The whole rankings for maps would be for example like :
Rafis and ...
(The accuracy would be an average of the both players and the Score would be the sum of the both players score, for example 95 % and 100 % gives 97.5%)
The Rank would be calculated ofc by the score and accuracy.)
I think adding this mode would give a whole new experience to the game.
Plus maybe some new mapping rules for this mode ? [Also since this is tag. hard maps need to be ranked, such as 12* etc.] [OR make a whole new Difficulty calculation system for tag.]
Also you should make a whole new BAT etc. for Tag
I don't really see this whole thing getting integrated, but hey
it's just a theory... a osu! theory !