When I've mirrored repositories with the intent of keeping them in sync, I've typically given them the same UUID. I don't know if that has much impact in practice, since I think working copies tend to stick to one of the mirrors (either the RW master or the RO slave).
The philosophical question here isn't whether the ID is "universally unique" but what it's identifying. Is it identifying the repository content or the the container in which the content is held?
