I didn't realize you could do that with only 2 x 8-pin connectors. Aren't they only supposed to be able to produce 150W? So that's 150+150+75=375, nowhere near 500W.
https://forums.geforce.com/default/t...-pin-vs-6-pin/
the mobo provides up to 75 watts through the PCIE lanes, a 6 pin PCIE plug is rated up to 75 watts, and an 8 pin PCIE is rated up to 150 watts.