Plug 2 fans into the splitter and the the single end of the splitter gets plugged into the case fan header on the motherboard. This will allow the motherboard to power and control the fan speed. I'm guessing that the power supply cable you're referring to is a molex connector to power the fans directly from the power supply (this doesn't get plugged into the motherboard). This would make the fans run full speed ALL the time. Probably not what you want to do.