Lemmy's cross posts are separate posts that just happen to link to the same thing. so only replies to the original post would be sent with the current design.
that said, i severely doubt Lemmy will gain anything from this. publishers will not be sending out their posts to any communities, and i highly doubt they will expose any fep-1b12 group actors you can subscribe as a community.
kbin/mbin with it's ability to follow users may work better, assuming people test their federation with software other than mastodon, and accept any of the interoperability bugs as actual bugs instead of ignoring them. (lemmy itself is no stranger to this: the fact that users and communities can share the same username break quite a bit)
people in real: hey how's it going