Can I jumper across amp speaker outs to a 2nd amp's spkr outs?

What actually goes wrong, if anything?  Speakers generate BackEMF which amplifiers withstand.  Why should signal affect another connected (but unpowered) amplifier?

And what would happen if both amplifiers were playing different music?

Please only answer if you actually know

I would also like to know actual outcome.  My great nephew asked me about possible liabilities connecting 2 amps and using 1 at a time a few years ago.  I strongly discouraged doing so but could not supply a good answer as to why other than the certainty that at some time somebody would goof and power up both amps.

I have also seen this question posed before on Agon with no specific answer.

Can anyone here with electronic chops give a good explanation?

Bill

Because you would be backfeeding one amp's signal into the other's outputs, a treatment for which no amp is designed. Amplifier output signals can be considerable; if you're listening to inefficient, low-impedance speakers really loud, you could see 40-50V and tens of amperes during transients.

@devinplombier You are correct and that was part of my answer to my great-nephew but it doesn't answer the question as to what might happen and why.

Bill