Craig,
See this thread, about the 6th post down, where Tom Stage has posted color diagrams of the best way to wire up such an arrangement. It references a different brand of DCC system, but the concepts are exactly the same.
Be sure you skip the first few posts with the black and white diagrams (they were practice! ), and refer to the ones in color.
To do it right, you will actually need a 4PDT switch, which isn't too expensive or hard to find.
HTH,
Steve
The safest way to wire the programming track is through a DPDT center-off toggle switch. Wire the toggle switch center terminals to the programming track. One set of end connections on the toggle go to the programming track output on the DCC system and the other connections go to the track output on the command station. This way, you can run a locomotive onto or off the programming track if you so desire, and the programming track can even be an isolated section of mainline trackage.
Just be sure you keep the DCC system's main track power output wires the same as they go through the toggle switch. If an engine stalls when you try to run it onto the isolated section, the wiring needs to be reversed.
Craig North Carolina