Question A Z390 motherboard should switch to x8/x8 PCI with second graphics card, right?

Aug 23, 2019
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When I add a second, identical 2080ti graphics card and Nvlink to my Asrock z390 system, the cards show up in x16/x4 mode, not x8/x8 mode as expected, so I get no SLI.

The only other thing I have installed on the motherboard is an .M2 SSD which I've tried in two different slots.

I've tried doing a clean install of the drivers, but this seems like a hardware issue, right? (The motherboard should put the slots in x8 mode when two cards are installed.) Is there some way to force x8/x8 mode?

Windows 10
32 GB RAM
2x EVGA RTX 2080ti hybrid
EVGA NVLINK bridge
Asrock z390 Phantom 4 motherboard
Intel M2 SSD
1TB SATA HDD

Thanks!
-Donald