In some ways, it's like splitting your brain in two, with one half playing guitar and one half singing. McCartney demonstrates this to its max. I think if you know the song well enough, one or the other may be so automatic, that you can even improvise on the other part. And it's also a little like learning to play without looking at your hands, especially your left (if you're a righty) hand - you gotta force yourself to do it until it's second nature.