I have only done a bit of Symfony 2 but the vast majority of project I have seen are using separate bundle for the Admin backend, such as the sonata project
It provides a better separation of concerns, as backend actions are oftenly related to CRUD, when frontend are usually more displays and users managements stuffs (depending on the goal of your project though).
To go further you can also already find bundle similar to the admin backend generator module of symfony (v1), like the symfony2admingenerator
Hope it helped a little.