There's been an ongoing debate about whether communities should combine or stay separate. Both have significant disadvantages and advantages:

Combine:

• Network effects. Smaller communities become viable if they pool together their userbase. Communities with more people (up to a point!) are generally more useful and fun.
• Discoverability. Right now, I might stumble on a 50 subscriber community and not realize everyone has abandoned it for the lively 500 subscriber community somewhere else, maybe with a totally different name.

Separate:

• Redundancy. If a community goes down, or an instance is taken down, people can easily move over.
• Diffusion of political power. Users can choose a different community or instance if the current one doesn't suit them. Mods are less likely to get drunk on power if they have real competition.

This isn't an exhaustive list, but I just want to show that each side has significant advantages over the other.

Sibling communities:

To have some of the advantages of both approaches, how about we have official "sibling communities"? For example, sign up for [email protected] and, along the top, it lists [email protected] as a sibling community.

• When you post, you have an easily accessible option to cross-post automatically to all sibling communities. You can also set it so that only the main post allows comments, to aggregate all comments to just one post, if that's desirable.
• The UI could detect sibling cross-posts and suppress multiple mentions of the same post if you're subscribed to multiple sibling communities, maybe with a "cross-sibling post" designation. That way it only shows up once in your feed.
• Both mod teams must agree to become siblings, so it can't be forced on any community.
• Mods of either community can also decide to suppress the cross post if they feel it's too spammy or not suitable for cross discussion.
• This allows you to easily learn about all related communities without abandoning your current one. This increases the network effects without needing to combine or destroy communities.

Of course, this could be more informal with just a norm to sticky a post at the top of every community to link to related communities. At least that way I know of the existence of other communities. I personally prefer the official designation so that various technologies can be implemented in the ways I mentioned.