Possible? Yes. Plausible? No.

 You'll have thermal issues, and you may have to remove the ssd cages for clearance but why do it? SLI is dead and mGPU only works in what, 5 games?

Essentially no value for gaming, because the 3080 does not support nvlink, there's no sli bridge to connect them.

For mining, this is not an ideal setup.  

Even though there are two PCIe x16 slots, Dell never sold the Aurora R12 with two video cards. This is shown in the online Aurora R12 Setup and Specifications PDF, "Number of cards (maximum)" =

They won't sell dual cards for the r12 3000 or 6000 series Nvidia/AMD neither has dual card support unless you run mGPU which is well pointless