This is unusual but not that rare. When I was younger I wrote half a hard-boiled detective novel with a friend, alternating scenes every week or so over email.

This is unusual but not that rare. When I was younger I wrote half a hard-boiled detective novel with a friend, alternating scenes every week or so over email. We even had a rule that we wouldn't discuss the direction of the plot, which lead to some amusing twists. Plenty of books and more that one author (sometimes credited, sometimes not). Things like movie tie-in or spin-off novels might pass through several hands to ensure brand-consistancy before being published. This group seemed to have a lot of fun (and success). I am slightly jealous.