I have Tor running as a system service, but I also want to use Tor Browser Bundle to browse the web.
This means (I think) that I have two copies of tor running, one as 'debian-tor' and one as 'user'. Because I have two different Tor instances running, I likely have two different guards that I always use at the same time. This is bad because an adversary can now track my physical movements more easily by observing my guards and correlating connections made to them.
How do I use Tor Browser Bundle with system tor, without letting the tbb-tor connect to the network?
This is a bad idea for several reasons, especially with Tor Browser 4.5 alpha or newer as the circuit use behavior between vanilla (eg: a system tor) and Tor Browser's bundled tor instance is substantially different in several ways.
You will use two guards, but for two different types of connections and may not use both simultaneously.I don't see much advantage for your adversary. If you want to be more quiet, you can still add some noise to your connections by adding random tcp trafic (watching video on youtube...).