It's good to leave a time-gap between cross-posts. That way the post in the community with less activity doesn't get de-duplicated out of existence.

It's also better to post to smaller communities first, because then it will be visible in the cross-post list once you post to the bigger one.

It's kinda like a soft repost, and posts usually do better posted one at a time, since cross-posts don't share votes even though they kind of combine into one post, if posted around the same time.