@Gordon_Freeman There isn't a crosspost button, but if two threads are similar, they'll show as crossposts of each other. I'm not sure exactly what the criteria are, but from what I can gather, two link threads will show as crossposts on Kbin if they have the same link. Otherwise, they need the same title (and maybe the same photo?).
@Gordon_Freeman Agreed, though it would be cool to have a button that you can click to make a crosspost. Maybe it'd send you to the thread creation page with certain things filled out to make it a crosspost.