Piefed's implementation of this idea is a good place to start imo

Comments from crossposts are organized into sections according to each community and you can easily read a community's sidebar by clicking on the icon next to it (red arrows). I think those sections should be collapsed by default, this way it would be harder for users not to notice that these are comments from a different community.

Here is something to consider, sometimes one link is posted multiple times to the same community, how would you deal with that?

Edit: When a user wants to reply to a comment from a crosspost there should be a reminder/indicator that this is a comment from a different community or something.