You can cross post to different communities via the two double/overlapping squares that appear under the headline of your original post.

1. Create and post original post in community A
2. Go to your posting and click on the two overlapping squares under the headline of your original post. Title, text, body. will be automatically filled in.
3. Choose community B in the drop down menu you want to cross post to and publish it.
4. Repeat steps 1-3 for additional cross postings to other communities.

There will be a remark and link to all cross posted posts/communities the same URL has been posted to.

Hope, this helps :)

once you post to one community, if you press the little "copy" icon you can x-post to another community