I don't think you can necessarily use the the presence/lack of interaction between multiple writers as being the delineating point between roleplays and novels, after all, not all novels are written by individual authors. When considered in this context, what's the difference between a 1x1 RP and a novel written by two co-authors? 

I think it might have something more to do with intent, specifically how the resultant work is intended to be consumed. Generally roleplays are intended to be consumed in the process of their production, whilst novels are consumed in the wake of it. But that's only a half finished thought, I'm sure someone can find examples with which to disagree with me.