You could put the other side into something that looks visually like a thread. Each side will have a different history. They will also have a marker that says the history was split here and click here to view the other side.
If you can come up with a good design for what a client that does that should look like, and what information it would need from the server to do that, please do write it up and publicise it. I think ultimately something like that has to be the solution, but it would have to be actually fleshed out into something that's possible to implement.