you can use Upto 15v on a pc fan the 15v is more speed for the fan and is still safe to use but if you want to use more fans like you atm you using 2x12v fans so you need 24v to power them so use a multi volt power plug preferable for them 2 use a 15v as the 12v is split into 6vlt for 1 fan 6vlt for the other fan so with the 15vlt you will have 7 1.2vlt per fan
Black/blue Neutral
Brown/Red Live