You have to switch the power supply on via the ATX connector.
The signal "Power Supply On" must be connected to ground to start the power supply.
Ok cool, will use the old wire in green / black to get it to start with motherboard. But otherwise all ok to use two PSU for one system?
Minor detail: Most EVGA PSUs have one color for the entire cable, so you'll probably have to check a pinout diagram carefully to use that trick.
Some of their PSUs also come with a "power-on self-tester," which is essentially a fancy plug just for this purpose.