I have been looking at and thinking about this and to be honest I don’t see any way within WP and ACF to do what you’re looking for.
If I had to do this I would likely abandon the idea of having bidirectional relationship fields. Instead I would store the information in a way that would make it easier to do a reverse relationship lookup.
See my most here https://acfextras.com/dont-query-repeaters/
In this case I would create a standard WP field that held every relationship (post ID) added to the post where your repeaters are located. Then I could use this single post mata field to do the reverse query.
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