You should NEVER short the output of an audio power amplifier to the output of the other audio power amplifier because they will probably be destroyed due to the differences produced by stereo.
Why connect the outputs together? You will not have more loudness because an audio amplifier has an extremely low output impedance of 0.04 ohms or less so it is a voltage source with an extremely small voltage loss.
If you want to change stereo into mono then you do it at the inputs of the amplifiers, not the outputs. But then you might be shorting together the outputs of the signal source that might destroy them.