Configuring two sound cards on traktor 2.5

Hello everyone!

I am mixing on traktor pro 2.5 with a VCI-400 connected to a macbook pro running os x lion.
I usually use the jack master output to connect my vci-400 to my hi-fi system but it has some problems and i won't have it for a couple of days (maybe weeks), until it gets repaired.
I thought it would be no problem to continue mixing but i realized that when i set "built-in" in the audio setup of traktor, there is no sound anymore coming out from the headphone output...
I looked a bit on the internet and i figured that i could use the AudioMIDI Setup application to combine my computer's soundcard and my vci-400's soundcard to make it one and use this setup in traktor's audio setup.
The thing is that even when i've configured my soundcards like that, i can't get the sound out for my headphones.
I didn't find any tutorial on how to do such a thing but people say that it should work so i wanted to know if someone knows how to do this.

Thanks!!