I believe more and more people will escape to lemmy sooner or later, and I already see 3 different communities for the same thing (selfhosted) here on lemmy. Time will tell which one will be the most active, but lets assume all of them will be equally active and the desire emerges to combine the two communities about the same thing, is that something possible or intended to be possible with federated services?
The down side of that would probably be duplicated content. Like if some major news happens for a topic reposts can already be really annoying and usually need moderator action to combine threads. Then there’d be that times however many communities exist for that same topic.