This icon makes a cross-post, it shoud be used, because it combines them in the feed if both are in the same view (often in New), but it probably should be automated, at least if the link and the title are the same, so it also works if someone doesn't use the button to cross post.
Example:
This way you still are only one click away from each communities comments.
Lemmy should, however, find all federated duplicates and offer you to cross post an existing one when such a case arises. It would fix many of these cases.