Both would keep exiating. I imagine it like a federation on the community level. Gaming@kbin.social could federate with gaming@lemmy.world 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.

Secondly, the issue of the same posts being displayed for days under “Hot”. There is already a pull request for this, so it has been fixed and just needs to be implemented:

Adding the "hide" feature from reddit would be an easy fix/stopgap solution. I had a similar issue in reddit when one of my multis included a less active community, so I'd just hide the posts I was done with.

How would merging communities across different instances? Would one be chosen as the new primary, and the other goes away?