To respond to questions about implementation, I was thinking something along the lines of communities are federated with each other.

Some workflows:PostingI want to make a post to c/tech, so I post on tech@lemm.ee, my post then gets federated to the other instance and shows up there. (Servers would want to add de-duping for people who are subbed to both)

Deleting my postSimilar to above, it gets marked as deleted on both

Mod action:Mod on lemm.ee removes my post, the event and reason get federated. The other instance could be set up to follow the action or have a mod action to do anything with it.

But anyway, for now I'll try to see if there's a way to group communities, but I don't think Sync for lemmy has that.

The reason I was thinking this is because this part of federation actually makes things harder. And then there's the aspect of "where do you post this", bc sometimes the community is bigger on another instance, so you cross post just because it's essentially the same thing, just somewhere else.