I think they probably stuck by their official song sharing for this very purpose: to get double credit. McCartney and Lennon are weaker than Lennon/McCartney. Do we even know how much they collaborated after a certain point? By Sgt. Pepper at least they might’ve had less to do with the other guy’s final product than did George Martin.
You would be surprised how much we know about their collaboration on a song-by-song basis.
I think I read somewhere that one would take the beginning of one song and add the “middle 8” or somethign like that
If you listen to a lot of Beatles you can almost see where one person wrote one part and another wrote the other part- Lennon often adding the heavy edge rock to it- I dont know.. I am just guessing
I do this with my daughters all the time. I cannot play piano for anything (at least not very well) but when THEY are playing, I can accompany them like crazy- inventing new sounds by ear almost as if by magic