Sounds interesting! I have a few questions about this idea.
1) What setting does each story take place? Each character may be completely different from one another, but all characters must following the rules set forth by the setting.
2) How different are the separate character plot lines? Assuming that all characters are going about their day in the same setting, then how do the same events that occur in the setting effect each character? Each character's reaction may not be the same, but how do the events coincide? For an example, you can't plunge the entire world into civil war in one plotline and have the other characters be completely unaffected by this turn of events. All the characters are doing something at the same time other characters are doing something else. How do one character's actions eventually effect another? This gets especially confusing when one character's plotline branches out into several different events and choices. If on one plotline the character becomes a messiah but in another plotline the character becomes the harbinger of destruction, how would this effect all the other characters, who also exist during the same time? If each character exists in the same timeline, you could be headed for a collision course disaster! But hey, perhaps the other characters are going to one practically different planets and utterly oblivious to it all. But if the other characters are going to be oblivious, then it has to be for a good reason. Are you planning to set certain guidelines for each co-author as to what events must occur and effect each character, or is every single character going through a completely different adventure?
3) How do you plan to complement each author's writing style? How do you plan to throw a dozen different writers into the mix? Some writers may complement each other, but it would ruin the overall composition of the story if one writer were to go into a huge tangent with a literature style reassembling Shakespeare's sonnet with little choices while another author were to write a story the way one would blog with a near infinite amount of choices that carry little weight and were to end after a few pages. This could be an interesting experiment and it could work out fantastically if you play it out properly, but it could also clash and burst into flames.
Hm, perhaps this is a little more than three questions, but you still get my drift, right? Although the idea of a co-authorship sounds simple, it's actually a very complicated idea ESPECIALLY in a storygame-like medium where the plotlines may branch out indefinitely and get tangled up in each other in an utter mess.