Communities on different instances about the same topic should have the option to essentially federate so a post on one appears on all of them and opening any of them shows you the comments from all of them. This way when lemmy.world is down its not a big deal because posting to any news community federates to all of the communities instead of barely having people see your post. Federation could be decided by the community mods and the comments can have a little “/c/communityname@instance.name” on it so you know which community the comment was originally posted on.
If the mods can agree on policy, there is absolutely no reason to have two communities. Shut one down and use the other.
Edit: can someone explain to me what the difference between synchronizing two communities and subscribing to a federated community is? I mean, that’s exactly the point of federation.
That system makes the instance a single-point-of-failure for the whole community, which has been a big problem lately. If communities could easily be multi-instance they would have redundancy. That seems like a good reason to me.