Route to both amps, use two widget buttons in a group and inverse the value of one of them. Map each button to the bypass parameter of each of the amps. It should work.
I did reverse the widget value scale to get the desired outcome when the light was “on” yes.
I’ve also found that if you have more than a 4 channel mixer, for example an 8 channel mixer, that the on/off toggle will jump from the lowest pair (1-2) to the highest pair (7-8).
You can of course change this by changing the min/max of the widget value curve so that the button toggles between pairs 3-4 and pairs 5-6 for instance if the curve is set to min ~38 and max ~74.