The quick-and-dirty:
1. Remove connector, tag wires for reference (to be safe, but they ARE color coded).
2. Find a 12v relay (power rating shouldn't matter, you will be passing less than 500mA.
3. Solder leads on to said relay, and shrink wrap bare metal.
4. Connect wires 14 and 15 (or 16 and 17 if you have a 24-pin EPS12v PSU) on the "slave" power supply to the switch portion of the relay, and the coil leads to the 12v +/- leads on the "master" power supply:
[code:1:f66819b1c8]
| | (---------- (coil +)
(switch +)----o \____ | | (
(switch -)___________| | | (---------- (coil -)[/code:1:f66819b1c8]
Mind you, polarity doesn't matter, just an indication as to what's what.
5. Splice all remaining wires from "slave" PSU into the corresponding wires on the "master" PSU.
6. Plug in your devices (estimate, and balance the load, if possible). Give it some juice.
Now, an explanation:
The "master" PSU, when powered on, will trip the relay coil, which will activate the switch, and ground the PS-ON lead, in effect "fooling" the "slave" PSU into thinking it is receiving the power-on signal from the motherboard (PS-ON will be pulled low when it receives the signal). The motherboard will then send the PG (power good) signal to the power supplies to let them know that the voltage has stabilized, and that it is OK to continue output.
2 reasons it has to be done this way:
1. The PS-ON logic will not likely be rated to pull both PS-ON leads low (out of ATX spec), which will either not power on either PSU, or fry the motherboard. Either way, no es bueno.
2. The PG signal is somewhat complicated; it must be signalled between 10 and 300 msec after power on, not simultaneously, and not too latent. Again, the logic circuitry is not rated for 2 PSUs.
So... sorry about going all Mr. Wizard on y'all...
Hope that helps!
Cheers,