I am currently working on a new project where I wanted multiple apps (frontend (fe) and backend (be)) to have the same exact appearance, and without having to duplicate the code in both template/layout.php files. So what I did is chose the fe layout.php file as the master template, then made the following changes:

// config/ProjectConfiguration.class.php
public function setup()
{
    ....

    sfConfig::set('masterTemplateUri', sprintf('%s/apps/fe/templates/layout.php', sfConfig::get('sf_root_dir'));

    ....
}

// apps/be/templates/layout.php
<?php include sfConfig::get('masterTemplateUri') ?>

Now I can share the same template across both fe and be apps and only have to make code changes in one location.