This was a book in the Dr Seuss/PD Eastman format (i.e. about that size, not very long, line drawings with only a few colors) about a boy who starts noticing footprints on his ceiling and it turns out there is a boy who lives upside down. They make friends, but they can only go some places together. I remember a scene at a dock, for instance, where one was on top and the other was underneath. It has to have been published by 1964 or so.