Before our users can adequately troubleshoot, they need some more data.

* What PSU shipped in your Aurora R12, 550W or 1000W?
* What was the originally shipped video card in PCIe x16 (x8 electrical) slot 1?
* What video card did you add to the PCIe x16 (x8 electrical) slot 4?

I have two Aurora R12 with 3090's. Can I take one of my 3090's out and put into my other R12 with the 3090, essentially having Dual 3090 R12 system? (Is this possible with just the 1000 watt PSU?) And if so, would this be worth it to increase my R12's performance? (I would put in a 3060 into the empty one to use as my work computer)

I do not believe it will fit in the bottom slot.

And even if it fits, your CPU will become the bottleneck. 

2 x 3090 is overkill unless you plan on gaming at 4K with really high frame rates, but in that case you would need the best of the best CPU not to bottleneck the video cards.

The heat from 2 of these in an Aurora case would be another issue, and you would at least need the 1000 Watt power supply option and that might not even be enough.

Then there is the power draw from the PCIe slots, that might cause serious issues with these boards.

Any update on getting the 2 Gpu's working? Is it possible? I'd like to squeeze in two 3080's or 3090's if possible. 

For gaming, with the 30 series I thought that "SLI" was basically dead, and offered no real performance benefits.

For mining, sure, you would want to use as many cards and as many SLI slots as possible. You need to check your power supply and work out if it can actually support the required power, and you need to experiment with PCI risers and probably a side panel open with some A-Team bracket to hang the second card from. I very much doubt it would fit two 3090s inside the case, without also inadvertantly creating a pottery kiln at the same time with all of the heat build up in what is already a notoriously air-constrained case.

So question 1: do you have adequate PSU to power it

question 2: can you actually fit two cards into the case

question 3: can you get the software / bios to support it

I would think you can get it working, but if your target is gaming performance: total waste of time.

For the latest Ampere GPUs (3000 series) SLI is not supported on anything but the 3090.  You can't even get a bridge for any other 3000 series card.

I can use a second GPU. But there is a problem with the second PCIE slot. Whatever thing I plug into it, any video card or a wireless adapter, a process named “system interrupts” will take more than 10% CPU usage. I tried to reinstall OS and drivers several times. The problem persists. It might be a hardware defect or just a bad bios Dell provides.

Because of the front panel wires I can't even plug my 3080 gpu into the second slot in the bottom. This slot is just for show really sad...

I figured it was going to be a very tight fit.

With 2 video cards that close together and both having +300 Watts of power usage, it's going to get very toasty inside that case even if you could make them somehow fit.

Has anyone got this working?

I can connect two 3080's to my PCI-e X1 slots (slots 2 & 3) via risers, and they work fine. 

But my hope was to leave 1 3080 plugged into slot #1 (PCI-e X16) and connect the 2nd 3080 to slot 4 (the 2nd PCI-e X16) via a riser.  But any time I plug a riser into one of the PCI-e X16 slots, the system doesn't recognize it.  I've tried disabling and enabling, and updating my drivers.