Yes, 2 separate 8 pin PCIE cable from the PSU to the 12 pin addapter.Oh interesting! So if I'm using the 3080 FE 12 pin adapter, it would be two separate PCIE cables, one for each 8 pin? I will try this!
I'm not sure if it will make the system boot, but as the PSU is below the recomended spec, 650W not 750W and only one cable, the GPU may not be getting enough power to show a display, this may be the reason you dont see a POST as not enough power to the PSU.
Thats my thoughts on it, knowing me I am wrong, but it's worth a try.