Good point.  I wonder though with a lot of "duos", if the lopsidedness is not more the rule, than the exception.   How much of the work is truly 50/50?  Probably never.  With some duos, the music is mainly done by one member, with the other person writing most of the lyrics, or one person writing most of the song structure, with another writing just the bridge.

I know with Lennon/McCartney they often wrote in sections with the earlier tunes,  where say the verses might be Paul, and the bridge John.    Prime example is We Can Work it Out....the "try to see it my way" part was Paul (optimistic, kinda reflecting his view of life), then the bridge was "life is very short, there's no time for fussing and fighting my friend"  (how typically Lennon).   Other songs,  in Beatles' repertoire were written exclusively by either John or Paul, but all went published as Lennon/McCartney.