I did just realize that I could solve this by just directly connecting the 12V PSU to the 12V physical relay. That would be simple, but it would also mean that if the SSR controlling my 12V PSU were to fail closed it would close the relay contact and put power on the contacts for the SSR controlling the heated bed. It's highly unlikely they would fail at the same time, but I'd like to find a solution where the SSR controlling the bed heater can only get power on its contacts if the Duet thinks the printer should be on.

My board is:

1. Duet 3 Mini 5+ ethernet
2. Duet 3 EB 3HC
3. Duet 3 Expansion Mini 2+
   (I have so many boards because I have 4 steppers on my Z axis I would like to do auto bed leveling and I want to have 3 extruders in the new build.)

Background (probably not important):
I am about to install a duet board and as part of this, I'm overhauling the safety on my 3d printer. One of the things I want to do is install a physical relay in front of the SSR that powers my bed heater direct from 120v AC. I am doing this because I've read that SSRs tend to fail closed, and having my 700 watt heater turn on by itself when my 3D printer is in soft power off would be "very bad". The thermal fuse would probably stop a fire, but I'd like to have another layer ahead of the fuse anyway.

My original plan was to add a relay with a 5V coil before the contacts of SSR that controls the heat bed. That way I only get a click when I turn on the printer instead of every time the bed adjusts the temperature up. I would also be able to control both relays (the SSR that turns on 12V power and the relay that controls AC power to the contacts of the bed heater SSR) from the same PS_ON pin from the duet (at the moment the 12V PSU SSR is controlled from GPIO pins on a raspberry pi).

The problem with that setup is two things:

1. I can easily get a relay with a 12V coil that does what I want, but one with a 5V coil will take longer to get
2. Rationalizing my impatience, I also think it will be less dangerous to my Duet board if I get a crossover failure in the relay and 120V AC gets fed back to a 12V rail instead of the 5V rail.

My solution to that is to use 5V on PS_ON to turn on the SSR that controls the 12V PSU and then 12V from ... ?somewhere else? to close the 12V physical relay. I am very open to other solutions to this problem. I do not do 3d printer design or electronics design by trade or even very often, so you will not hurt my pride when you tell me I'm doing it all wrong

My actual question:
Is there a way to make the M80 command trigger a PS_ON pin at 5V and then trigger a different pin at 12V and leave it triggered until the printer gets an M81 command?