Brad Tipper

// Don't show posts of the same type as current as options
function filter_relationship_query( $args, $field, $post_id ) {
  $current_type = get_post_type( $post_id );
  $type_array = array_flip( $args['post_type'] );
  unset($type_array[$current_type]);
  $args['post_type'] = array_flip( $type_array );
  return $args;
}
add_filter('acf/fields/relationship/query/name=field_name', 'filter_relationship_query', 10, 3);

// Save link between projects and testimonials
function bidirectional_relationship( $value, $post_id, $field ) {
  $field_name = $field['name'];
  $field_key = $field['key'];
  $global_name = 'is_updating_' . $field_name;
  if (!empty($GLOBALS[$global_name])) return $value;
  $GLOBALS[$global_name] = 1;
  if (is_array($value)) {
    foreach ($value as $post_id2) {
      $value2 = get_field($field_name, $post_id2, false);
      if (empty($value2)) {
        $value2 = array();
      }
      if (in_array($post_id, $value2)) continue;
      $value2[] = $post_id;
      update_field($field_key, $value2, $post_id2);
    }
  }
  $old_value = get_field($field_name, $post_id, false);
  if (is_array($old_value)) {
    foreach ($old_value as $post_id2) {
      if (is_array($value) && in_array($post_id2, $value)) continue;
      $value2 = get_field($field_name, $post_id2, false);
      if (empty($value2)) continue;
      $pos = array_search($post_id, $value2);
      unset($value2[$pos]);
      update_field($field_key, $value2, $post_id2);
    }
  }
  $GLOBALS[$global_name] = 0;
  return $value;
}
add_filter('acf/update_value/name=field_name', 'bidirectional_relationship', 10, 3);