Sync 2 relationship CPT fields in ACF that are connected. Example: Actors (Relationship Field) in a Movie (CPT), and Movies (Relationship field) an Actor (CPT) has been in.

acf-sync-fields.php

<?php

function acf_sync_field( $sync_field, $value, $post_id, $field ){

	// vars
	$source_field = $field['name'];
	$source_global_name = 'is_updating_' . $source_field;
	$sync_global_name = 'is_updating_' . $sync_field;

  // get this value early before anything is updated
	$old_value = get_field( $source_field, $post_id, false );

	// bail early if this filter was triggered from the update_field() function called within the loop below
	// - this prevents an inifinte loop
	if( isset( $GLOBALS[ $source_global_name ] ) && ! empty( $GLOBALS[ $source_global_name ] ) )
		return $value;
	if( isset( $GLOBALS[ $sync_global_name ] ) && ! empty( $GLOBALS[ $sync_global_name ] ) )
		return $value;

	// set global variable to avoid inifite loop
	// - could also remove_filter() then add_filter() again, but this is simpler
	$GLOBALS[ $source_global_name ] = 1;
	$GLOBALS[ $sync_global_name ] = 1;

	// loop over selected posts and add this $post_id
	if( is_array( $value ) ) {

		foreach( $value as $post_id2 ) {

			// load existing sync data
			$value2 = get_field( $sync_field, $post_id2, false );

			// allow for selected posts to not contain a value
			if( empty( $value2 ) ) {
				$value2 = array();
			}

			// bail early if the current $post_id is already found in selected post's $value2
			if( in_array( $post_id, $value2 ) )
				continue;

			// append the current $post_id to the selected post's 'related_posts' value
			$value2[] = $post_id;

			// update the selected post's value
			update_field( $sync_field, $value2, $post_id2 );

		}

	}

	// find posts which have been removed
	if( is_array( $old_value ) ) {

		foreach( $old_value as $post_id2 ) {

			// bail early if this value has not been removed
			if( is_array( $value ) && in_array( $post_id2, $value ) )
				continue;

			// load existing sync data
			$value2 = get_field( $sync_field, $post_id2, false );

			// bail early if no value
			if( empty( $value2 ) )
				continue;

			// find the position of $post_id within $value2 so we can remove it
			$pos = array_search( $post_id, $value2 );

			if( $pos === false )
				continue;

			// remove
			unset( $value2[ $pos ] );

			// update the un-selected post's value
			update_field( $sync_field, $value2, $post_id2 );

		}

	}

	// reset global varibale to allow this filter to function as per normal
	$GLOBALS[ $source_global_name ] = 0;
	unset( $GLOBALS[ $source_global_name ] );
	$GLOBALS[ $sync_global_name ] = 0;
	unset( $GLOBALS[ $sync_global_name ] );

	return $value;
}

functions.php

<?php

/* Example Usage */

add_filter( 'acf/update_value/name=relationship-field', 'bidirectional_sync_relationship_field', 10, 3 );

function bidirectional_sync_relationship_field( $value, $post_id, $field ) {
	return acf_sync_field( 'relationship-field', $value, $post_id, $field );
}