The feature request:
https://codeberg.org/quokkau/Pavlova/issues/3

About your idea, let me try to explain:
When you make a post in PieFed, and presumably to Pavlova, there's a form you need to fill.
This form has fields like title, tags, link, etc. which you fill to publish as a post, similar to Reddit.
And also similar to Reddit, it has different types of forms for different types of posts.
If you want to post, e.g., an article from PC Gamer, you'd usually pick the link form. If you just want to ask or say something, you pick the discussion form. And so on.
Your post would be your own words, so linking elsewhere would already be hardly justifiable.
And if you make a discussion post, you can't cross-post.
If you make a post in one of the two meditation communities and then link it on the post to the other, I can think of 3 problems.
First, there wouldn't be any other communities to cross-post to.
Second, it could be seen as offensive to the second community and any tangential ones you cross-post to, as you'd be leeching the given communities by indirectly saying "go to this community to interact".
Third, it could be seen as spam.

As community-based fediverse instances often group similar posts together, personally I'd suggest to just manually make the same post as a discussion post on both communities. Or pick the bigger of the two, either by subscribers or monthly active users (MAU).