From my understanding, if you simply repost a link that has already been posted to another magazine, then both threads reference each other in the comments section. These references include the # of comments, votes, boosts, magazine location, user who crossposted etc. You are able to change the title.
This functionality also works with threads only containing text. I'm less certain about this, but I imagine the body text has to match exactly. I haven't really tested beyond this one example.
Pamasich