If the 2nd PSU works fine when it's connected to a motherboard by itself, and you're only having problems with it when it's connected as a secondary unit to a mining rig — then you're probably connecting it the wrong way. Just need to figure out what it is you're doing wrong. For starters, I'd just ditch any sync devices and simply jumpstart the 2nd PSU by shorting the wires on the 24-pin connector.