The two of them started out as collaborators, and obviously developed a joint approach to songwriting, but by the time they had become famous, almost every one of "their" songs was really the composition of one or the other. Early on, they had decided to maintain the "Lennon-McCartney" songwriting identity as a business decision to split the royalties equally, even as they were beginning to work more and more separately at creating songs.

A number of their early hits, written as they were just achieving fame in the UK and before breaking out in the states and going worldwide, feature two distinct parts -- e.g. a verse by Paul and a bridge by John. As time went on, though, they wrote more and more separately.

Just being members of the same performing unit, of coure, assured that there would be some continuing mutual influence upon each other. Certainly, their work right after the breakup showed each one's individual charactersitics more blatently than they had appeared back when they were Beatles.