I’m surprised that you managed to misspell Jerboa twice in a row

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.

src: https://github.com/LemmyNet/lemmy/blob/63d3759c481ff2d7594d391ae86e881e2aeca56d/crates/api_common/src/post.rs#L61

I don’t even know how to cross post on the normal website