I think it depends. Not entirely sure though. Now given the context of Lemmy right, you can certainly post to instance C from both A and B. Even if your instance (either A or B) will not fetch messages from each other (due to not federating) you can always go see the messages through instance C (not logged in). That probably works for all defederation scenarios but that is also a very loosely defined interaction as both parties will have to do the same.

A diplomatic answer I know.