Actually, for a crossover, I wonder how well it would work to have each person write something from the POV of each universe/main character? That might be really interesting as a collective work, sort of pushing even further what someone else described as working for Xander/Spike. Of course, in that case, what I'm imagining has a bit less seamlessness (is a bit more seamy?), so that you can get some of the flavor of each universe in the writing.

But I do think constant communication is the only way to do it. You have to be writing the same story.

(I am not the person to do this project with, as I don't do SPN. But good luck!)