From a user front-end standpoint, just collate all posts with identical links and then make a tabbed system for comments. Lemmy.ml comments are on this tab, kbin.social comments are on this tab, etc etc. Seems like by far the easiest way to present it without (accidentally or otherwise) force-federating all of the source material. This could even pretty easily ("easily", yeah I'll get right on that) be done within the app if not done in the lemmy/kbin source code directly.