I love collaborating, providing the other party is a total pro--and I've been lucky there.
First, we "story-storm," and it's GREAT to throw out ideas in a room where the word "no" is not allowed.
Second, we put the action on index cards and figure out the order, then I'll type out the outline.
We divide it up according to what scenes/chapters we'd like to write.
In one case he couldn't bring himself to write a nasty murder scene, and I was not in the mood to do a love scene. We traded. I'm now able to write bloody slaughter with the best of 'em, and he's discovered a talent for writing dang good nookie.
Third, I only have to write half a book.
Fourth, if a reader doesn't like anything in the finished product we each blame the other for putting in that bit.