I am noticing how people really wish to shape the story their own way, and I wouldn't mind seeing how each person interprets each function. so I propose that you write what you wish, and if you disagree with what the last person said and would like to change it, then do so but tag the upper left corner as to where your splitting the story so we can follow it.
as in the example of them staying together or splitting, whoever posted the first can leave it alone, and the person who wants to change it just marks the top (A), and anyone who continues off that branch puts (A) at the top, and if there is another branch off a branch, then alternate letters/numbers so (A1)
EDIT: To symbolize the split (in story) of the two groups, you could use a symbol rather than number/letter, like "#" so the story would go (A1a1a1#) where the symbol represents which group of characters your dealing with inside the stories tangent that deals with two groups.
to resolve the issue of their being a branch off the first story after the initial branch of (A), then it would start with a number, and not a letter, but would alternate the same (1A1A1A)
EDIT: Also, if someone misses their que to split, but would still like to, thay can put the post number they would wish to split off from before they label their new tangent
this way we can create more of a branching story that people can shape as they will, and if someone doesn't like something they can just change it without destroying someone elses story.