Well they aren't set up to use both at the same time. They are set up to use the touchpad when nothing else is attached, and then when you attach say a mouse, trackball, etc., it will use that and not the touchpad. It shuts the other off since they are both doing the same thing.
So as long as the mouse is connected, it would be the device that is used and when you disconnect it, then it will go back to using the touch pad. They aren't made to both work while both are connected. Just like if you were to connect an external keyboard, or external speakers, the ones attached to the laptop would not work until those were disconnected.