@woody
Without seeing a wiring diagram of what you plan its hard for me to say if it will work.
But powering an expansion board like a 3HC with a different PSU is A-OK. One test machine I have has 3 different PSUs (one at 25V one at 30V and one at 42V)
If the expansion board is CAN controlled, then its important to know that power isn't transferred via CAN so you can power each expansion board with its own unique power if you so wanted.