OP didn't say force. OP specifically said allow.
YourHuckleberry
This is a really good idea. Multi-instance communities would not just provide content redundancy, but also some load balancing. Each multi-instance community would become it's own little CDN. Duplicating the data across instances does pose a problem of bloat, but I think the benefits outweigh the risks.
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.
I went to college with a Spanish guy, Milo I watched this exchange.
Girl: "What kind of Spanish are you?"
Milo: ...
Girl: "You know, like Mexican or Colombian or Puerto Rican?"
Milo "No no, I'm from España"
Girl: "But like, which country did you grow up in?"
Milo: " España...uh Spain"
Girl: ...
Me: "There's a country in Europe called Spain, its the place where spanish originated, like England is where english originated."
Girl: "Duh"
Me: "That's where he's from."
Girl: *suddenly realizes how dumb this whole exchange has been and dies of embarrassment.
You'd think science would have already figured that out right? Oh yeah, they did. A century ago.
https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/
Don't TLDR, actually read it, but it basically says, "we've done the math, it's definitely us."
People think we're smarter than dolphins because we invented skyscrapers, digital watches, and war, while all the dolphins ever do is goof off in the oceans having fun. Dolphins know they're smarter for the same reasons.
We're going to be the first species to go extinct due to stupidity. A dude the other day told me that the guy who invented the weather channel says that global warming is due to Earth's orbit not being a perfect circle. I don't know how to combat that level of stupid.
That's an important and valid concern. What if the community federation could allow mods on your instance to ban users from other instances? You'd not see that user's posts or comments when viewing a community from your instance. The downside is that your mods would have more work.