the ART website don't make it clear but probably only thing you can record from ART USB Mix is its main out. i saw some review said so. if you playback your tracks they'll be blended into main out, and that's the signal passed to S1.
if you can record stereo from the USB Mix, then simply swing all the previous tracks to left or right and record only other side will do what you need. i'm no sure it does record stereo, though.