We've done things differently with different works, For our children's book, we had a complete outline. Each of us would write one section each week, and we would revise when we met. This method worked well. By the end, you really couldn't tell who had written what. (en)
Our overall process changes from book to book. Usually while I'm working on another project, Mary will come up with an idea, run it by me for my input, then get on with plotting and research. Nothing's ever set in concrete. No two books follow quite the same procedure. (en)