Combine left and right channel at speaker level
You should not connect the two output amps together, as you will be effectively shorting them out. That may destroy them, although many amps will have protection circuitry and survive. If you want to double the output power, the best way is to buy a second speaker. Using 2 speakers will also mean that you are able to play stereo - at least, that's if you don't combine the 2 inputs.