@Jurigag: I guess it's not about routing nor handling primary request, it's about "sub request", a term which does not exist in a HTTP world, but closest thing to this Symfony approach would be Disaptcher in Phalcon.
Since Silex is based on the HttpKernelInterface, it allows you to simulate requests against your application. This means that you can embed a page within another, it also allows you to forward a request which is essentially an internal redirect that does not change the URL.
Basically, they inject entire HTTP request into another 'route', which is then handled by some other controller/script. In Phalcon, you don't even need to do it as there is a shared Request service.