This appears to be the scariest topic relating to Symfony2, as after a week of searching and testing, I am still unable to find an answer to this.
In short, I am buildin
Basically Fabien is right, there's no reason to have more than one application, if you really have a need for a different application it's probably a different project. Bundles and libraries can be easily shared like any other bundle you see on the web. Then you can have the small part of the set up belonging to each thing you call "app" in the app part of each project. If they share the entirety of the code then it's just a matter of configuration hierarchy for each sub-domain, which could be your case considering you want to share some part of the config.
Symfony has many ways of allowing you to reutilise code which are very nice, but the framework is not meant to have many applications, if you want to try to hack it, go ahead, but then you're not using the framework anymore. And that's why you can't find examples, not because it's scary, it'd not be that hard to modify, it'd just be ugly, IMO.