2 GPUs 1 system, nvidia not showing up

[Manmeyco]

So I just got an AMD RX 6700 XT, to upgrade my system which currently has a GTX 1050 ti, with a R5 3600 and 32gb of ram on a Asus Crosshair VIII Hero X570. I have 2 PCIe Gen 3 m.2 SSDs as well. I want to keep my nvidia gpu in the system, to run things that need CUDA cores, and to add extra grunt for things like folding@home. (I have 2 monitors, but they both will be connected to the AMD.) My issue is, the nvidia gpu does not want to run in the 2nd pcie slot, as in it doesn't show up at all, in device manager or even the bios.

Here's what I've tried so far:

• I did DDU with just the nvidia in the top slot, and downloaded the latest drivers for each gpu (but didn't install).
• Put the AMD in the top slot, moved nvidia to the 2nd. Only amd showed up, installed amd drivers.
• Shut down and swapped nvidia and amd, restarted, both showed up, installed nvidia drivers.
• Shut down, swapped back (amd top, nvidia 2nd), both showed up and played fine.
• Opened up 3dmark, immediately got a BSOD, caused by GPU-Z according to the error code. Restarted, nvidia gpu is gone.
• Swapped again (nvidia in top slot), both show up, re-install nvidia drivers.
• Swap back (amd top), nvidia not showing up.

And that's where I'm at now. I've tried disabling and enabling resizable bar, running with just the nvidia in the 2nd slot, and they don't work.

For some reason, nvidia doesn't want to work in the 2nd slot. I can try swapping back and forth again, see if it fixes it, but it's annoying.

 

I have 2 sides to my question– why doesn't the nvidia card want to work in the 2nd slot? Is it a motherboard limitation? PCIe lane allocation? But then why does the amd work (at least, it turns on) in the 2nd slot?

And the other side is, can I run the amd gpu in the 2nd slot? I don't think that's an ideal setup, but if that's what it takes I might do it. I'm just worried about things like resizeable bar not working, programs defaulting to the nvidia gpu (because it's in the main slot), and PCIe lane allocation, because I've seen videos where GPUs in an 8x slot perform worse (the 2nd 16x slot is electrically an 8x).

 

Later today I'm going to try swapping them around again, but I don't have time now, so I figured I'd ask to see if maybe there's something I'm missing.

[manikyath]

some motherboards REALLY suck with PCIe power delivery, so if both GPU's are rather dependant on the power delivery from the slot, they may see a rather big dip on the 12 volt rail.

[Manmeyco]

3 minutes ago, manikyath said:

some motherboards REALLY suck with PCIe power delivery, so if both GPU's are rather dependant on the power delivery from the slot, they may see a rather big dip on the 12 volt rail.

I hope that's not the issue - my motherboard is $400!

Practically I doubt it's a power issue, as my motherboard is an X570 that's made for overclocking (beefy VRMs and such), and I just got a new 850w Asus PSU to handle the new card, and I made sure to use 2 separate cables for the 2 8-pins on the gpu.

How would I isolate a power delivery issue?