dual music output on air

i've a macbook air, since there is only one audio out on it, i got an external usb sound card that outputs to a 1/8 audio port, i can make it play fine, my question is,

how to get different audio streams onto each different audio out at the same time, ie, a headphone cue and sound out on the external audio device as if to dj.

any thoughts on this?

thanks