Here is how to show posts, that have at least one common parent with the current post.
Let's say we have two CPTs: Events and Producer,
and they are Related -
Producer is parent CPT, Events is child CPT, Many to Many Relation
There is a Relation meta key - we will need that.
On the Single Template of Events, place a Listing Grid, which shows Events.
In it, set two Posts Query items:
- Meta Query
Relation meta key IN %related_parents_from|producer%
- Posts & Author Parameters
Exclude posts by IDs - %current_id%
Then, this Listing Grid will show only Events, that have at least one common Producer with the current Event:
(all Events are shown for the reference)
Hope that helps!