It's a common misconception from our customers that flipping the CH 1-2 Select switch to the center to turn on both channels simultaneously means that you'll get a signal from both channels at once through one instrument.
However, this is not the case. While both channels are on at the same time when doing this, you will only get signal from the channel your instrument is plugged into (EX: if you're only plugged into CH 1, your tone will sound like what settings you have CH 1 set to). This is intentional, so you can run two instruments at once out of the same amp.
If you were looking to run both at the same time as a way to blend tones, you can run your instrument into both channels with a splitter cable like many do with the Aviator Cub.