There also need to be tools to merge two communities on separate instances, or move them.
The issue is not the tools, more the people. I contacted mods of !android and !askandroid a while ago to see if they wanted to merge, both sides wanted to stay on their own communities.
There are plenty of other examples, usually a large LW community, but with a more active non-LW alternative. LW wants to keep their community open, and the non-LW doesn’t have much power besides showing they are more active.