Combine stereo audio amplifier outputs
This is possible in some amplifiers and it's called "bridging" or a "bridge tied load" configuration. You need an inverting stage in front of one of the channels to make it inverting. The input signal is then split into the two channels and the output is taken as the potential difference between the two channel outputs. You have to be careful here because since the maximum voltage swing is twice, on paper, it looks like a quadrupling of the power is possible; it would be foolhardy to assume that this can actually be obtained.