From http://symfony.com/doc/current/cookbook/bundles/override.html :
Routing is never automatically imported in Symfony2. If you want to include the routes from any bundle, then they must be manually imported from somewhere in your application (e.g. app/config/routing.yml).
The easiest way to "override" a bundle's routing is to never import it at all. Instead of importing a third-party bundle's routing, simply copying that routing file into your application, modify it, and import it instead.
So, the solution should be to (again) import the DemoBundle routes from Some\Custom\Bundle routing.yml: