yeah gpu passthroughs basically have to have a gpu that isn't already assigned to the current OS. You have to blacklist the pci group that has your second gpu on startup (which can sometimes have other devices on it which complicates things and is also the reason I couldn't do one myself even when I had 2 gpus), then set up the VM to be able access that pci group itself and then run the VM. It's kinda complicated with each hardware setup comes different combinations of problems/solutions so no single guide on it's own will work perfectly but it's also not the worst if you have the basic idea down.

There's also ways to do it with only one GPU where the VM basically takes over control and while you lose access to your main host desktop/OS making it less useful than a normal VM, I'd imagine it's still more convenient than restarting your whole PC and maintaining a dualboot with windows. I haven't looked into it at all though because I'm profoundly lazy.