On those specs you could easily hook up an emergency battery system that employs one diode and one sealed battery for each ECU that will maintain minimum voltage/current for each of the four ECUs for over an hour (if that's how long you want to spend playing with the gear/waiting for battery to charge), for a total weight penalty less than 2kg and a cost less than £50. Certification of course would be a different question.
What a ludicrous design. Adding documentation procedures is not a human factors solution, it's a recipe for more accidents. If it's the preferred solution to prohibit starting both engines on ground power, then the design itself should prohibit starting both engines on ground power...