Behringer UCA202 & Macbook
I can use it independently as a sound card, which is fine, but what if I wanted to use it for just headphone monitoring and my internal core audio for playback. Is this impossible?
Nope. You need to use one sound card that has at least two independent stereo outputs. Can’t use more than one audio device/sound card in Traktor (or any other similar software AFAIK) at a time.
Using more than one sound card would be almost impossible to get working reliably anyway, since all cards have different latencies, bus types, etc…