Built-in Output As Master, and Soundcard as Monitor With Traktor 2?

Hey guys, so basically I'm looking for a very small portable midi controller to bring on trips and stuff just to pracitce, and I might get the Numark DJ2Go.

So I was wondering if there is anyway I'd be able to use the builtin ouput of my macbook pro as the master, and then use my external soundcard as the monitoring output for headphone cues. I have a mixtrack pro kicking around and I'm trying to use an aggreggate device to get the setup to work, but it's giving me 3 aggreggate outputs in the Traktor 2 audio setup window.

I tried putting the two aggreggates as my master and the 3rd as my headphone cue (and vice versa), but even though the headphones work when I switch the headphone cue on, the problem is both songs are playing at once on the master as well (laptop built in speakers). Anybody have any clue how to do this? Thanks!