BAD idea!! The output impedance of a power amp is a fraction of an ohm, so do you really want to connect both of your amps to nearly a dead short? Depending on the amp design, when a modern amp is turned off, it likely has a protection relay that disconnects the speakers from the amp output. BUT if you have an older amp, or maybe an inexpensive amp without a relay, then the output transistors are connected whether it is on or off. It really depends on the design.
And mistakes happen... what if you accidentally turn both amps on?
Simply a bad, bad idea!!