JT,
I had two Numark USB mixers (the low end ones), similar to what you have (battle size). They used to have a different model name, although I sold mine off some time ago, and can't recall.
Tons of problems with those things, along with a few customers who had them as well.
Many times, only one channel would work, or both decks would end up outputting to the same channel on the mixer.
My guess at the time, was there was a quality control issue, and the DACs on the mixer may not have had enough spacing or something between the etch lines on the board. I don't know if that's what's happening in your particular case, but if they are still using the same boards, it might be...