Twig extend template on condition

Try this one:

{% extends intro == 'false' 
    ? 'UdoWebsiteBundle::layout.html.twig' 
    : 'UdoWebsiteBundle::layout_true.html.twig' %}

Idea taken from here: http://jorisdewit.ca/2011/08/27/extending-different-layouts-for-ajax-requests-in-twig-symfony2/