Projects are different. Usually we do an outline in advance. Sometimes, one of us does a first draft, and the other guy rewrites it. Sometimes, we alternate sections -- either chapter by chapter, or maybe one guy writes about one character, the other writer does a different one. Action sequence or exposition. Depends on several things that tend to vary from book to book.
Each collaborator is also different. Sometimes one of the writers is senior and directs things. Sometimes the partnership is more equal. Generally, somebody is designated as the writer who has the last pass; otherwise, you could ship stuff back and forth forever and never get done. Depends too, upon the deadline for the publisher. Usually each writer gets a pass and one of them does the final touch-up.
With my daughter, I was the senior writer. She did most of the work, and I touched it up. With Michael, the first books we did, he was the senior writer, and that's how it went. Later, after we were more on a par, that shifted.
If I didn't enjoy it, I wouldn't do it. The biggest drawbacks are two: One, is that you don't get to do it all your way; you have to allow room for the other writer to put his or her opinion in and sometimes you don't agree. You have to let that go.
Plus you only get half the money.
I like doing my own stuff better for those two reason -- nobody tells me what I can write, and I get the full advance, less my agent's cut.
Somewhere in a drawer I have a collaboration, an (unpublished) short story I did with Bill Gibson years ago. One of these days, I'm gonna put it up on eBay and see how much it's worth.
