There may be another option and that is to use an electrically operated circuit breaker. Remember, I'm thinking out load here.
With these types of breakers, a contact closure is provided to the breaker and that contact MUST be powered from the switched side. Once the contact is closed, the breaker unlatches (sometimes by heat) and the power is removed from the equipment.
SO, it gives you a nice power switch. Alarm contacts on the breaker can be had too.
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That out of the way, the other issue is the 50 A that we could be dealing with. Somehow, one needs to make those wires safe and with that means a connection up close. Fused up close would work.
A 555 timer can be used as a comparator and that could be a place to start.
By assuming that you will always have a functioning supply, we are assuming that only one will fail at a time.
So, think of a device that hooks to the output of the supply, with the inputs fused as soon as possible and provides a contact closure to a externally mounted breaker.
Many supplies have a NOT SHUTDOWN pin and it's possible that could be exploited.
When having protection and two power switches, you may not be able to get the devices on unless you turn BOTH on together.
It takes some time for a aux trip breaker to function.
Again, thinking out loud.
So, yea, it's not too hard to provide a signal that the power supplies are good, BUT you have to provide a start-up mechanism (delay) before it functions. Usually you also provide a delay before it reacts too, so you don't get nusense trips.
Mounting part of the device inside of the supply solves some problems - namely high current lines exposed. It might require two internal devices; one for each supply.
Some things my crystal ball can't see. i.e. do the supplies have a power switch? What's the mechanism of turn-on? Is turning on two together OK?
One note, is that operating power supplies in series, you should have protection, A big high current diode reversed biased at the output might be all you need for a more reliable system. The big guys recommend these when the supply is operated in certain ways, but they are not usually included.
If it were me, that's what I would do first.