i can't tell you exactly, but from some very quick research it seems like the way finishers work are:
first key press -> works like a regular object, it determines your accuracy and whatnot
second key press -> it gives you bonus points, but it doesn't affect accuracy. it should only give the points if the key is pressed within the timing to hit it(depending on od). if theres a note right after you might lock into that note causing a miss and no extra points.
i've heard of some players pressing finishers in the same way they'd do 1/8 dd or kk. so it's something you can do if you want.
NOTE- i'm not 100% sure on this