There are a couple of ways to do this.
You can simply short the GREEN PS_ON to any one of the black 'COM' pins (ground), this will render the power supply 'always on'.
If you want it to turn on with the main board's connected PSU, wire the same green PS_ON pin from the main board PSU to your second PSU's PS_ON pin, along with one of the adjacent COM (ground) pins. This is how the cable-saurus adapter works.
This adaptation is handy if you're not comfortable soldering on new wires (recommended), but you can try it out just securing it with electrical tape (not recommended, definitely not permanent)
OR you can always go buy one or more of these:
Either way you are good to go with two (or more) PSUs on one main board
