You absolutely 100% have to use the PCIE 6+2 to power the cards, that's where they get their power from, they won't run without it.
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I hope this helps.
Yes, it does help! Thank you very much! You did understand what I'm asking.
BUT! While my AMD GPU's have 8-pin slots on the top, the Nvidia ones only got 6-pin slots. That's what I'll have to connect apparently. Just noticed that.
I think I'll just hook them up via 6+2 using only 6 pins, feed the riser via Molex and check if the cords get hot. If any problem occurs that'll be a reason to create another thread (after prior research)