you can but don't do it.
Most PCIe cables are AWG16 (better ones are AWG14) and they're rated at 13 amps. 13A * 12V = 156W. That's the maximum power that's safe to draw using those cables. If you use 1 cable for 2 cards you're asking for trouble, those cables might catch fire.
not like this.
PEIe cables with 8Pin was 3x12v, so 13*12*3=450w.
Are you sure AWG16 can run with 13A?
I check some data , AWG16 max is about 6A.
so one PEIe with 8pin can only drive 6*12*3=216w.
so this is why 1080ti must 8+6pin or 8+8pin