@roxie ok uh here's a bunch of things to try
* sometimes the bios just has an option to choose the default GPU. sometimes its visibility is gated behind UEFI mode. sometimes it's gated behind UEFI with legacy CSM disabled. sometimes there just isn't one at all even though the idea of multihead in a PC was made popular by friggin windows 98
* pretty sure there's a linux kernel cmdline thing to manually tell the passthrough stub driver to keep a given pci address out of the hands of the gpu driver but i can't remember it off the top of my head
* ... is swapping the cables an option?