you could check it using a multimeter, make sure your PSU produce the right voltage, or else you can try using Relay Adapter Connector to connect two PSU.
I always count max GPU power consumption (TDP), also another part such as a Riser power, Processor, RAM+SSD (even they only need a small power), additional fan (if we are using it) to pick PSU.
You could try to plug four GPU, two cards powered by PSU that you are using for the mobo, and the other two cards using the other one PSU. Riser power using PSU no 2.