I have finally booted up the mobo with a RTX2070 and have installed the latest BIOS V2.13, and have installed Windows 11 Pro. When I install RTX3090 into the second PCIE slot, Windows 11 crashed saying device not working properly. I have also installed Nvidia Studio Drivers but the same problem happened. Before the crash, I can see in the Device Manager, the RTX2070 got a warning message saying Windows is uninstall the driver (code 21). The Display shows RTX3090 but when click on it, it has not info except the message "scan for new hardware /devices"
Any advise is highly appreciated.  
 
 
edit by CoolGTX - (fixed with DDU) added to title

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PSU? brand, model, (size) Watts, age, are you using the Stock cables that the Mfg supplied?
CPU?
Other hardware?
Type of cable being used to connect the GPU to the monitor
 
 
1) turn off fast boot in the MB's BIOS ..... with only 1 GPU in your PC. 
 
     - The power has to be removed from all AC sources when changing any hardware
     - use separate power cables from PSU to EACH GPU power plug
 
2) turn off fast startup in Windows, if it is still an issue

bdary
If this is a new OS install, did you install the needed drivers like chipset etc...

starting with the Basics is a good idea, too many times people overthink these issues & forget to ask if the "basics were done first"
 
 

if you look at the PCIe slots in the BIOS settings, does it identify the 3090? Does it call out PCIe gen4?
 
It sound like the 3090 has an issue from what you've posted so far. I'd suggest trying a firmware update on it, but if you can't get into Windows without it crashing while the card is installed, I'm not sure how you'd do that.
 
what kind of 3090 is it?

were you able to confirm that you installed the motherboard's chipset drivers (found here)? It may also be helpful to install Intel Driver Assistant to get the latest intel updates. Also, as Cool said above, disable fast boot.

kk700
Dear All, I have finally get the system boot up with 2 RTX3090. Basically what I did was to use UDD to uninstall all display drivers and reinstall Nvidia drivers and reinstall all the drivers from EVGA. 
 
Thank you everyone who has replied to this thread 

Good to hear you got it resolved. 

DDU & clean install of Nvidia driver got it working
enjoy your new build