As an update to what I suggested earlier, use either one of the soundcards to take a stereo signal from the computer into the mixer, and then use the mixer's USB output to feed the result back to the computer.I actually have two soundcards in the pc plus the behringer mixer one. How would I configure it so one soundcard feeds to the mixer and then out of the mixer to the second soundcard? Is that possible?
You will need a correctly-configured lead for splitting the stereo headphone output of a soundcard to two separate mono channels. It should go from a 1/8" minijack to a pair of TS 1/4" plugs, e.g. the Hosa CMP-153.