<?php
function bidirectional_acf_update_value( $value, $post_id, $field  ) {

	// vars
	$field_name = $field['name'];
	$field_key = $field['key'];
	$global_name = 'is_updating_' . $field_name;

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

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

	// loop over selected posts and add this $post_id
	if( is_array($value) ) {
		foreach( $value as $post_id2 ) {
			// load existing related posts
			$value2 = get_field($field_name, $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 (use field's key for performance)
			update_field($field_key, $value2, $post_id2);
		}
	}

	// find posts which have been removed
	$old_value = get_field($field_name, $post_id, false);

	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 related posts
			$value2 = get_field($field_name, $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);

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

			// update the un-selected post's value (use field's key for performance)
			update_field($field_key, $value2, $post_id2);
		}
	}

	// reset global varibale to allow this filter to function as per normal
	$GLOBALS[ $global_name ] = 0;

	// return
    return $value;
}

add_filter('acf/update_value/name=amp_post', 'bidirectional_acf_update_value', 10, 3);