I had thee ideas on how it would go about,
1. We would each write a story for the other, and offer the chooses at the end, so the writer would be doing the massive lions share on their own, but the other would just pick which way it goes and the writer would go the next page, if this was the way it went i think each would have a story that the other was working from, so both get to write and choose, just in different stories, they could even start out the same.
2. It would just run like a choose your own adventure book, we do a normal RP that we pick out (adult in nature of course) and then at the end of each posts you would put 2-3 "for gwen to suck off the troll to get over the bridge go to page 65" and the other would just do the next post as if that was the chosen page.
3. (if i had to but not really the idea anymore) a RP were the characters life is in a CYOA book and they are following along on page.