I've been on systems where multiple developers were trying to develop on the same system at once. I've also seen teams trying to do it systematically. It scales basically to two developers, sitting across from each other. Three, again, physically colocated, on a good day. Even if they're working on completely separate tasks, you hit "compile" and it's a complete mystery what's going to happen. It's not even stable if you do nothing and just hit "compile" again.

Beyond that it's insane. You do not want that in your version control system, as something built in, working all the time, across your entire team. It would be a massive anti-feature that would nuke your product.

Again, anyone thinking this sounds like a totally awesome idea, I strongly encourage you to try out the simple version, availablbe right now, of just "five or six people editing the same source code checkout" right now, before betting a start up on it. I guarantee a complete lack of desire to productize the result if you try it for a week or two.