This will look better later.
Start by using your favorite terminal text editor. In this example, we'll be using nano.
sudo nano /etc/default/grub
We'll need to add our secondary GPU's Device ID's to following flag.
vfio-pci.ids=
Example usage:
vfio-pci.ids=10de:06cd,10de:0be5
When you're done make sure you use grub-mkconfig to update the GRUB Bootloader. Restart Required.
sudo grub-mkconfig -o /boot/grub/grub.cfg