I only have AMD RX gpus, and they use the 8 pin plug for the GPU. You should be fine with one modular plug per card. I could technically plug in two cards on each of my plugs... however, I have read that that is not recommended so I have not tried it. Then you connect the sata power plugs to the PCIE powered risers.
You understand nothing about electricity or circuits and how they work.
There is a reason why some cards have an 8 pin and a 6 pin socket. The 6 pin just supplies 3/4 more power on top of the 8 pin and is used for more power hungry cards such as the 1080. First lets get it all straightened out as to what all those wires are for. The top row are all 12vd+ wires that are all COMMON to each other. The bottom row is DC Ground. Or in other words the negative of the 12v power supply rail. Having each wire positive or negative goto its own pin that ultimately is soldered to the circuit board keeps all the current split up and going into the circuit board instead of ALL of the current going thru one or two or even just 8 pins. They are only rated for so much current flow. Exceed that rating and things get hot and melt. 10 amps going thru 7 pins (7 is half of 14, remember one half is pos and the other neg) instead of just 4 reduces the current flow in each pin of the socket. This also reduces the flow in each of the feed wires that lead from the plug in the power supply going into your PCIE power wires by splitting up the wires into multiple pos and neg wires.
to add some clarity;
The 8 pin connector doesn't add one more + and - line, but it only in fact adds two more grounds only.
Liken the need for 2 more grounds to having a fire hose; and you wanted to increase the pressure on the inlet.... so you want to beef up the inlet, whereas the outlet won't feel the stress the inlet does. Since electricity flows in a magical circle from negative to positive; you need to beef up the negative coupling. This isn't 100% correct; but good enough for the layman to understand.
I may have analogized that kind of wrong; but its pretty close to a good description on reasoning.
In short; if you are running a GPU off one cable; as long as the AWG of that cable can support the card's wattage (factoring in the PCIE 16x port's power draw), you can use both connectors on the one pcie cable to power the GPU.