Both would keep exiating. I imagine it like a federation on the community level. [email protected] could federate with [email protected] and when you view one of the two, you would see the contents of both as if they were one community. When creating a post you would still need to decide to which of both you want to post it, though. This would make sure that there are no problems in the case of "defederation".
That's how I imagine the whole thing. Not sure if there are any negative side effects I can't think of right now, though.