At one time when old jazz and show tunes were written by duos, this would make sense for the publishing to be split 50/50 as one would often write the music while the other wrote the lyrics. However, for music where there may be more then two writers, this is not necessarily the case. Perhaps you're thinking of Marillion. I believe it was stated in their Separated Out biography that when they were writing the follow up to Clutching at Straws, Fish apparently wanted 50% of the publishing as he wrote the lyrics while the rest of the band wrote the music. The band weren't too happy about this which also caused tension and was a reason for Fish leaving.
Wilton
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