I have 4 PCIe connectors on the PSU.
Each cable splits to 2 8 Pins.
I have 2 cables (4 x 8 Pin) to the EVGA 3090.
3 of them are in the card and one powers the riser.
I have 1 cable (2 x 8 Pin) to the Zotac 3090.
They are both in the card.
I run a Molex for the riser.
That‘s pretty hefty, I know.
Almost 70 watts from the molex and 230 watts from a single 8 Pin to the PSU.
The Zotac 3070 sits right on the board with the last 8 Pin cable and has got the 2 x 8 Pin in the card
Did you decide to do this yourself or did another miner give you advice?
You need to connect 2 RTX 3090 video cards and raisers (6 pins) to your 850 w power supply, and for the motherboard and RTX 3070 it is enough to use a 600 watt power supply and for this video card you can use a molex raiser (although it is better to use a 6 pin raiser)
Here is the solution to your problem.