You'll have safety Nazis come in but I've been running all year with just shorting the 2 pins on the cable, no add power adapter or anything. I was just using a paperclip at first but since every PSU is EVGA now I have tons of their "tester" adapters that short the 2 pins.
or just a one wire jumper between the main and secondary PSU's PS_ON line so they can be fully shut down if wanted.