[solved] RHEL 7.6 Server with GUI is only detecting one of two displays

Hello,
I'm running RHEL 7.6 on Dell Optiplex 7060 with Nvidia GeForce GT 730 and habe connected two displays via DisplayPort two the GT 730. But the operating system (using the nouveau driver) detects only one display.

Why is that? When booting a gparted live media I'm able to use both displays out of the box. Is the driver in RHEL 7.6 just too old? Or is there something I could do to get the second display working, too?

I've read that I could download and Install the propritary Nvidia driver. Since I have an UEFI system with secure boot enabled I need to sign the Nvidia kernel module so the kernel would load it. In order to do that I have to add the public key to the trust anchor of my operating system. Could someone point me to a guide explaining how to do so?

Best regards,
Joerg