Can you please clarify that? It would be really interesting to know a little more how that works. I think your two sentences contradict? I think that reflecting everything is actually what OP asked for. But how could it happen if the other instance is not notified about a change done by the first instance? It is stored in the database, yes, but there is no notification.
So just a db = stateless. Everytime you make a request it asks the database again - there is no in-memory state. Or is it?
Otherwise I really like the idea of a plugin. I agree that this would be the way to go and I also still think that SEO is not something you choose to do but somehow have to in order to get audience. Duplication would be a way to actually cut it. Even from the user’s point of view it might just be suspicious if I see something I wrote on one site mabye linked on another site I had no idea about, I would be really disturbed. Content duplication is typically a method used by low quality sites. That’s why it gets a SEO downgrade. But this would be more of a topic to Community category.