Communities on different instances about the same topic should have the option to essentially federate so a post on one appears on all of them and opening any of them shows you the comments from all of them. This way when lemmy.world is down its not a big deal because posting to any news community federates to all of the communities instead of barely having people see your post. Federation could be decided by the community mods and the comments can have a little “/c/communityname@instance.name” on it so you know which community the comment was originally posted on.
Even if we wanted to solve that problem, right now there is no way to cross-post on Lemmy. There’s a cross-post button, but it actually does a repost. I think we should think about that when Lemmy implements a cross-post feature in the first place.