code: public function split(ItemInterface $item, $length)
      {
          return array(
              'primary' => $this->slice($item, 0, $length),
              'secondary' => $this->slice($item, $length),
          );
      }
docs: Split menu into two distinct menus.
  
  @param ItemInterface $item
  @param mixed         $length Name of child, child object, or numeric length.
  
  @return array Array with two menus, with "primary" and "secondary" key