Say for instance two or three instances have their own version of a sublemmy for some niche topic. Is it possible to group those sublemmys together so I can view posts from each federated instance as if they were all on one site?
Sure, over time things might normalize and one instances sublemmy will probably reign supreme, but for now is this possible?
Pretty much exactly like that, yes. I know implementation would probably be complex with the way (de)federation works but it’d be a nice QoL feature imo.