Just wanted to post that I figured out my errors on this, and now have it working!
First in the filter it should have been
add_filter('acf/update_value/type=relationships', 'bidirectional_acf_update_value', 10, 3);
not
add_filter('acf/update_value/name=related_posts', 'bidirectional_acf_update_value', 10, 3);
referencing the relationship field type
The other issue was when assigning the second field_name and field_key I had a single =
if($field_name = 'artist_releases') { $field_name2 = 'release_artists'; $field_key2 = 'field_5bf7a2e215e5c'; }
if($field_name = 'release_artists') { $field_name2 = 'artist_releases'; $field_key2 = 'field_5bf79ed45b371'; }
should have been
if($field_name == 'artist_releases') { $field_name2 = 'release_artists'; $field_key2 = 'field_5bf7a2e215e5c'; }
if($field_name == 'release_artists') { $field_name2 = 'artist_releases'; $field_key2 = 'field_5bf79ed45b371'; }
This now works for bidirectional relationships with different names.
Thanks!
