I was able to use SLI on my Z390 board with an Intel i9-9900k pairing 2 EVGA 3090's w/ an EVGA NVLink. There were no issues at all. I recently got a 5950X and an MSI X570 GODLIKE motherboard, so I swapped those parts out(as well as the memory). Anyway, I got the system up & running and did a clean install of Windows 10. All drivers/updates have been processed and I successfully re-installed both 3090's separately(then attached the NVLink afterwards). Windows sees both 3090's just fine and is not reporting any issues with either card.
However, in Nvidia 'Control Panel', SLI is disabled. If I try to enable it, it attempts to, but then goes back to "Disabled" again.

 
I did notice at least in the BIOS, it's showing something that seems odd to me. Under the PCI_E1 & E3 lanes, it only allows me to choose "Auto" or "x4+x4":

 
Also, under 'Board Explorer', it's showing that both the PCIe x16 & PCIe x8 slots are both running at x8:


 
As you may see in the photos above, I specifically got this motherboard because it's the only X570 motherboard that supports the proper 4-slot spacing for 3090 NVLink(they have to be installed in PCI_E1 & PCI_E3). I also have NVMe SSD's installed in M.2 slot #1 & M.2 slot #2. It does not say anything in the motherboard manual about me not being able to do that.....
 
Does anyone have any idea on what's going on?
 
thank you

https://www.msi.com/Motherboard/MEG-X570-GODLIKE/Specification

Board advertises SLI compatibility, but no configuration of populating slot 1 and 3 allows either slot to run at x16. If you look at the expansion section of the details tab in the link above, your configuration should in fact yield x8/x8. The only alternative is x16/x0. As to why SLI won’t enable, that is not related to the link speeds you’re seeing.

I had this problem a few weeks ago while trying to benchmark. Try putting the DP cable into the secondary card. Get an imagine and then swap back to the primary. That's how I got it to work.

Oh and yeah, I did confirm that 2-way is 8x/8x(which according to "Board Inspector" in BIOS, it's showing that's what both cards are running as):

 
And according to the manual, I'm supposed to be able to click on "Maximize 3D Performance" to enable SLI:

 
but for whatever reason, it just reverts back to "SLI disabled".

^^^^ After years of running SLi this!!! As well as trying Display Port Cable in other GPU (to ensure they both work properly).
Also, try installing GPU Driver with both GPU's installed (including SLi Bridge). I noticed you said you installed GPU Driver with only one GPU installed.
 
 

https://www.reddit.com/r/nvidia/comments/kdro5w/3090_2_and_3_slot_bridges_are_here_coming_soon/

Get the A6000 3 slot NVLink bridge.

UPDATE:
 
I've tried running DDU in Safe Mode and re-installing drivers, re-seating cards/bridge, etc...   no luck!
Since hearing that Nvidia no longer supports SLI mode, I tried going back to an earlier driver from January. Magically.. SLI works again...  but!... once I update to the latest drivers, it becomes disabled again.
 
According to Nvidia, looks like SLI has intentionally been disabled:

UPDATE:
 
I ended up RMA'ing the EVGA NVLink and just received a replacement in the mail today. For the heck of it, I also bought the reference NVLink from Best Buy(also got it today). I've tried swapping back & forth between the 2, but it still gets stuck at "Disable SLI".
 
Re-size BAR is enabled, Above 4G is enable, everything else set to "AUTO". I also removed the M.2 drives I had in M.2 Slot #1 & #2 and just have 1 M.2 installed in M.2 Slot 3. Even ran DDU in Safe Mode and then re-installed the latest Nvidia driver again(466.27).
 
At this point I'm kinda feeling I just want to get rid of the 3090's and switch over to 6900XT's instead. :(

Do you have an older SATA SSD to install OS and see if that does anything?
Just to eliminate M.2 altogether. 
 
Hope you get it sorted.

Did you try without Rebar? I couldn't even boot to Windows with it enabled on my SLI 3090s.

nvm

Another member here sent me a photo showing that he has SLI Enabled with the latest Nvidia Drivers:

 
So glad to know that it is still possible to have it enabled. That and also nice to know you have it enabled with an M.2 in the 3rd slot. I have tested out each card separately so confirmed that they are both functioning. I will do some more tinkering right now....

Numb78

jd760
Another member here sent me a photo showing that he has SLI Enabled with the latest Nvidia Drivers:

 
So glad to know that it is still possible to have it enabled. That and also nice to know you have it enabled with an M.2 in the 3rd slot. I have tested out each card separately so confirmed that they are both functioning. I will do some more tinkering right now....

Have you any update on this? I'm stuck in the same situation, I'm trying to use old Nvidia drivers with no luck (up to now). Please let me know! Thanks :)

Turn off the resizeable bar, reboot and see what that does for you. With sli it might not like the rebar setup, or its just simply not coded for it in sli yet. Although, nvidia doesnt seem to keen on supporting sli much anymore. Did you do a cmos reset? That way it clears out all your bios settings and it will read whats installed and make new settings.

SlamFunk
I had this issue just recently with a customer machine. I found that enabling CSM and setting the graphics cards to legacy mode allowed me to enable SLI via control panel.
 
While operating in UEFI mode, the secondary card's core clock would reach 0 Mhz at idle. Legacy mode seems to prevent this ultra low power state from occurring. I believe this could be related, if not the cause of the issue.
 
I'm not an NV engineer of course, so perhaps my correlation is without causation.  Whatever the case, it does seem to work.

could flipping the switch to from normal to OC on the card fix that? or even boost lock within X1 tool?