Is it actually a problem? One of them will naturally "win" - they'll get the first chunk of people, then people will join that one because the people are there.
If we do get two vibrant competing communities it will be because they provide something different to each other in some way and then we probably don't want to lump them together.
I think multi-lemmys would be useful but I don't think we need them as a solution to the competing communities situation.