Sorry if my comments haven't been helpful. But I don't think the problem you're trying to solve is tractable. I'd be very interested to hear of a solution that doesn't rely on locking and is immune to conflicts.

You might fake it (and meet your stated goals) by maintaining a true master behind the scenes and syncing to two public slaves... (ie, forks you treat as peers, each with a "master" branch) but that still leaves neither of them truly "canonical" -- a designation of authority that would apply to the place where you'd resolve conflicts given simultaneous commits.