I actually have some experience in this topic. Several years back, I volunteered to step in and take over organizing a charity book. The book was a gaming book for an RPG company, and we had about two dozen contributors, both writers and artists. My advice is manage expectations and let writers do their thing. Here is what I mean: have each contributor including yourself discuss exactly what aspects of the project they want to do. Put it in writing if you can. Make an agreement on who will do what work. Maybe one of you will write dialog and the other writes everything else. Maybe one wants to only write first drafts and the other does the rewrites and editing. When Good Omens was written, Terry Pratchet and Neil Gaiman would take turns each writing one chapter at a time. Just have a plan and make it clear where responsibilities lie.
