Подсмотрено на просторах сети, после небольших правок стало готово к использованию:

<?php
    public static function cross_app_link_to($app, $route, $args=null)
    {
      /* get the host to build the absolute paths
         needed because this menu lets switch between sf apps
      */ 
      $host = sfContext::getInstance()->getRequest()->getHost() ;
      /* get the current environment. Needed to switch between the apps preserving
         the environment
      */
      $env = sfConfig::get('sf_environment');
      /* get the routing file 
      */
      $appRoutingFile = sfConfig::get('sf_root_dir').
            DIRECTORY_SEPARATOR.'apps'.DIRECTORY_SEPARATOR.
            $app.DIRECTORY_SEPARATOR.'config'.DIRECTORY_SEPARATOR.
            'routing.yml' ;
      $routeUrl = "";
      if (file_exists($appRoutingFile))
      {
        $yml = sfYaml::load($appRoutingFile) ;
        $routeUrl = $yml[$route]['url'] ;
        if ($args)
        {
          foreach ($args as $k => $v)
          {
            $routeUrl = str_replace(':'.$k, $v, $routeUrl) ;
          }
        }
        if (strrpos($routeUrl, '*') == strlen($routeUrl)-1)
        {
          $routeUrl = substr($routeUrl, 0, strlen($routeUrl)-2) ;
        }
      }
      if ($env == 'dev')
      {
          $path = 'http://' . $host . '/' . $app . '_dev.php' . '/' . $routeUrl ;
      }
      else 
      {
        $path = 'http://' . $host . '/' . $app . '.php' . '/' . $routeUrl ;
      }
      return $path ;
    }
?>

Можно положить во frontendConfiguration.class.php, или оформить в виде helper’а. После чего просто вызываем:

<?php
frontendConfiguration::cross_app_link_to('backend', 'module_name')
?>