If you could give the exact model of the graphics card that would help, but to me 750Watts running graphics two graphics card at full ( which is what any kind of crypto mining does ) is the bare minimum for your energy requirements, it may be shutting off one down so that you can keep the other running.
Concur. Your cards are pulling approx 600w collectively, leaving the rest to power your mobo and fans, which isn't enough quite frankly.