Two amps into one pair of speakers

There are inexpensive speaker/amp switch boxes available on Amazon. I got Bryan from https://www.zynsonix.com/ to build me a custom switch box which allows me to route two amps to single set of speakers and a sub. Added benefit, it has 4-pin balanced xlr taps for running hard to drive headphones. 

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!!  

Smoking in bed.

Why is this thread even on Agon, the 2nd amp for a gaming system?

The tube amp won't be hurt if off. Tubes cannot be hurt because it has plates and need electrons to move via heat.  Not sure what would happen with the MAC because of the back EMF through the transformers. . In any case like others has said it is very risky and a bad idea.