Changing settings on a Magic Trackpad - I can't find how to do this, only the built in one.

I've added a Magic Trackpad to my MacBook Air set up so that I can operate it in clamshell mode (closed) I can control the settings of my built-in track pad but not the Magic Trackpad. I have tried changing the settings in privacy to ignore the built in track pad, but it still doesn't let me update my magic trackpad, which is frustrating as it just cost me £120. Any ideas on how to do this? Ideally I'd like both my in built and magic track pad to operate using the same settings.