24 Pin Converter/Tricking the PSU

Didn't know if this should go in the PSU section or the motherboard section but i was wondering if anyone knew where i could find a 24 pin adapter that tricks the PSU into thinking it's connected to the mobo.Not sure what it's called actually but i wanted to get another GPU and i don't want to run my current PSU to it's max and i have another PSU laying around so i wanted to use that just for the extra card.Any help would be apreciated.
 

This psu doesn't have a switch, I just pull the paper clip

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How could it mess it up?Would it overvolt it since theirs nothing else connected to it but the GPU?
I have seasonic 520watt as my main PSU i'm using now.In theroy it should work with x2 6870's but i don't wanna chance it.I have my old Apevia PSU from when i first bought the PC.It's very shitty PSU but i'm sure it's enough to power one 6870.