This works for stereo amps and I have tried it as an interesting experiment: as Brian Eno suggested back in the 70s, one can place a small speaker behind the listener and connect it to the two positive terminals of the amplifier for a "third" channel effect. The resulting sound is the differential between the two channels. It won't harm the amplifier and places no additional load on the amp. One can put in-line a potentiometer to adjust volume of needed.
The results are interesting and do expand the sound field greatly.