They certainly have plenty of differences both on a technical and functional level. They're also focussed on separate (though similar) creative directions and they're both very much in development and not final. It could easily be years before either settles with something that proves itself to work well.
At any point, ideas might traverse the void. This has clearly happened several times over already, nothing wrong with it and while it helps an on the interface-ideological-level, it does nothing to help developers unite behind a common programming language or compositor.
I think it'll probably end with one solution "winning" (through progress on helpful features) and being adopted by the other camp but then again, there are few types of developers more stoic than FOSS developers. This might even take something like Wayland (essentially a replacement for X with its own compositing layer) coming along to mix things up.
I wouldn't put a bet on either breaking far away from the other. At least, not until the underlying tech changes.