One of the reasons PPAs work out so well for cooperation is that the packages tend to install without trouble on Debian systems, too.
I like to think that the distinct out-of-box experiences do not put strain on the relationship and that, like RHEL and Fedora, the two projects are not aggressively competing against each other for users. Roughly speaking, Ubuntu has fewer accessible developers and more users; for those who might want to participate in development by bug reporting, the former would be more important, while for many other people network effects from the latter would be.