You can turn the power supply on for testing by putting a paper clip between the green wire and any black wire on the motherboard connector.
I had to read the last post 3 times to find the relevant part of his reply where it was even
implied that he was having trouble finding out how to switch on the secondary power supply... I was like, "I don't think this guy's noob enough to not know about green-to-ground to power on", but at the same time I had to look over your post like, "wait, I've seen this guy before, he's not one of those non-English-speaking-so-they-don't-follow-the-thread-context noobs dropping replybombs* in threads... sooo...
... Then I found it:
Right now I'm waiting for a dual-PSU 24-pin connector in the mail (so that both PSU's are turned on when the whole computer is turned on)
Ahhhh... wow, he really DOESN'T know!
Dude, just jump green to ground. Look in the ATX connector for the green pin. "Green = PwrOn" used to be labeled on those power supplies but they got lazy... just to be safe I'd actually recommend powering a SATA hard drive/SSD off that power supply as well, just to give the +5v line something to do.
In fact, merging power plugs of both PSUs using a "dual" splitter cable is probably fuck'a dangerous - you'll then have two power supplies competing for voltage regulation. Don't put two independent voltage rails (e.g. "+12v" of one and "+12v" of another) together. They maintain their regulation by adjusting their load to match a certain level (thousands of times a second), so if there are two self-regulated supplies feeding into each other, one could blow the other up by backfeeding it - theoretically. It's likely never happened, but PSUs have
protection against that happening (that doesn't make it any more "proper"). Just jump the green pin to ground and call it a day
* - replybomb (n.):
Example:
joe: hay when i boot windoes with this new driver i installed it gives me an error saying the file is missing or currupt hurp?
jack: i seen that before somewhere... hit f19, stand on your head, hold mouse cursor by the tail and spin the display in circles and post the error code it gives you
joe: haha that was fun 0xDEadbeEF
jack: sweet, ok, you can fix that by inserting a slice of bologna in the cd-rom drive and singing glory hallelujah in b-minor.
joe: holy shit it worked
mark: U CAN BOOT TO SAFE MODE BY PRESSING F8 AT STARTUP AND DELETE ALL YOUR COOKIES!!