Apologies for submitting an answer without knowing for sure if this is the solution. I odn't have enough rep to comment yet.
Try adding this to your GRUB: nvidia-drm.modeset=1
alongside nouveau.modeset=0
. Make sure your BIOs settings are correct, too. If you installed in legacy or UEFI, boot in to the correct mode, turn off that strange setting, I believe it is called Secure Boot
in BIOs if you have it.
Did you follow proper procedure preceding a distribution upgrade? Prior to upgrade, safe procedure dictates the removal of graphics drivers and their repositories, then adding them again after the system upgrade reboot. If you didn't, what you might be able to do is enter terminal via recovery mode and try sudo apt remove nvidia* --purge
. Ubuntu should run fine with built in drivers. I would also definitely remove one card and boot up, then install drivers for both (if that's how it works, I'm not well-versed). As I understand dual card setups have a slew of issues, and running a unix based system instead of Windows sounds like a tough situation.