Since this helps greatly spreading posts in the Fediverse especially if they are initially posted in smaller instances or to find communities on similar topics.
Is there already an option and I have just missed it?
I believe any two posts linking to the same URL are treated as a cross-post, and that is discovered at instance-level. There's nothing more to it than creating two posts in different communities with the same link.
ETA: But to answer your question directly, there's no support for doing that as a single action with Jerboa yet.