The setting is tied to your instance account, because the backend delivers the results with the specified sorting. If you change it via the browser, it should be reflected in Jerboa.
Since account creation does not happen in the app, but requires visiting the instance's home page, where registration requirements, federation status and other peculiarities are plainly visible, I don't think this belongs in the app.
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