I like to connect my secondary powersupplies with a transitor.
It's super easy to do, and works great. When your main psu is switched on, it turns on the other powersupplies, when it turns of, they turn off. If anything ever goes wrong with your secondary powersupplies, it won't have any effect on your main psu or mobo.
What you do is connect the ground on both PSU's to the base of the transistor. Then connect the green wire (on off control) from the secondary supply to the emmiter of the transistor. Then connect the 5V rail to a resistor, and then to the collector. You want slightly more than 0.7V to be at the collector, so size the resistor according to the specs of the transistor. This may sound complicated, but it is only 4 connections. A quick look on google will explain what the base, collector, and emitter are.