The interface built in to the Q1202USB is 2 in / 2 out.  The signal being sent to the USB port is directly from the Main Mix fader.  This means that the signal going to the DAW is a stereo mix from the mixer, exactly like the signal going to the main outputs.  The recording of individual channels into individual tracks can only be done two channels at a time.  With one channel panned hard left and the other panned hard right, the two channels are effectively separated from one another in the main outputs, which means you can set the input of one track to be 1 and the other as 2 which will record the two channels individually.