How do you hook up two amps together
Connecting two power amps to a mixer/ single audio input
Connecting Multiple Amplifiers To One Source
How to Bridge an Amplifier: 7 Steps (with Pictures)
The best I have seen from many, with regard to this particular switch.
Separate switching of L and R channels is less convenient you need to flip 2 switches instead of one , but works just as well.
But, it's NEVER OK to connect multiple outputs.