PS_ON is a signal sent to the motherboard NOT the powersupply, the mobo then tells the powersupply to boot the main 12V rails. In ATX boards the power supply always supplies some power to allow the motherboard to boot on lan for example.
The only way i can think of is to split one of the wires in the ATX connector from one of your power supplies and have the wire connected to the unused ATX connector of the other one. This wire is the one the motherboard uses to send the signal to the power supply to liven up the main rails and boot the PC.
As for which wire that is, have a search for ATX specifications or similar, i also think this has been brought up on this board before.