Flipping the CH 1-2 Select switch to the center will turn on both channels of the amp simultaneously. However, it's a common misconception from our customers that this means you'll get a signal from both channels at once with one instrument plugged into one channel.
This, however, 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.
So, if you have the CH 1-2 Select switch flipped to the center and you're plugged into CH 1, your tone will sound like what settings you have CH 1 set to. Not a blend of both CH 1 and CH 2.
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.