Short Answer
You can’t.
At the time of writing this article (March 2025) you cannot select more than one community for your post.
How to post to a community
You can select one community when creating a Viva Engage post.
After selecting a community or storyline, you can add more people, but additional communities cannot be included:
See Also
Post as a leader to specific groups
Move Viva Engage conversation to another community