Coordination
“Johnny went to the psoe priret.”
That’s what you get when your right hand thinks “post office” but your left goes with “supermarket.” There might be occasions when you wish Johnny would go to the psoe priret and just never come back, but that way you don’t get the mail or anything for dinner.
In a partnership, each coauthor is like one hand on the keyboard. They must act as one in order to create intelligible prose. It’s not enough that that both pursue sensible ideas; they must pursue the same idea. They must both want Johnny to go to the same place.
Even if you don’t opt for a combined nom de plume, you and your writing partner must form a single virtual writer. No matter how solid your outline at the beginning (you do use an outline, right?) every story grows into some unanticipated shapes during its construction. Partners need to stay so in tune to each other that they can adapt to these ongoing mutations. It’s true that some authors don’t use outlines, and that doesn’t make them wrong. But consider: two writers each making everything up as they go can’t really be called a partnership; how do they even know they’re writing the same book?
This sounds like a nigh-impossible requirement, almost like a psychic bond. It’s really not that severe. Expect it to take practice, and talk to each other about both the project itself and also what you think it means to be a writer. Writers working together can be every bit as graceful and in tune as a figure-skating duo. They might not be as exciting to watch, but what they create is what matters.