Driver issue trying to get two GPU working together

Hi. I have a Nvidia GeForce Titan Z that I want to use for CUDA. I’m running ubuntu 18.04 (kernel 5.4.0-132-generic) and have downloaded the cuda toolkit 10.2. In order to use it only for cuda, I’m using a second GPU for the display, a Quadro FX 580. The problem is that launching nvidia-smi or nvidia-settings always return an error (the driver in use at this point was the 440 I think). Since both GPU doesn’t share a common driver, I thought I would change the GPU to a similar architecture, and I changed the Quadro for a more recent one, the K420.
They’re both Kepler, and can run with the driver 470. Thing is, no matter what, I can’t launch neither the nvidia-smi nor nvidia-settings, even for one card. It’s always complaining that the driver isn’t loaded (though it obviously is). Any idea of what’s happening ?

Edit 1 : nvidia-bug-report.sh result : bug_report

Please run nvidia-bug-report.sh as root and attach the resulting nvidia-bug-report.log.gz file to your post.

@generix I’ve edited the original post.

Please disable secure boot in bios.

I already tried, and very surprisingly, when I do that, I can’t boot anymore.

When you disable secure boot, please also make sure CSM is disabled.
Otherwise, you’ll have to set up module signing.
https://wiki.ubuntu.com/UEFI/SecureBoot/DKMS

Ok, I have a very hard time disabling secure boot. I end up in a login loop where even the ttyx are looping. mokutil is returning a non-informative error either when enabling or disabling validation.