Assuming you aren't going to be gaming on Linux directly (only in the Windows VM), PCI-E bifurcation can do this. Going down to x8 from x16 only loses you a few FPS, generally speaking, and something like Sliger's SM560 (http://sliger.com/products/cases/sm560/ - no relationship but I have a SM550 and am pretty happy with it) would allow you to fit a 2-slot Nvidia card for the Windows VM (recommended over AMD; Vega & Navi AMD cards cannot don't do pcie FLR or full reset properly so you'll have to do full machine reboot per VM restart if you're using one of those for Windows), and a 1 slot AMD workstation card for desktop usage. Note that you'll need a motherboard with BIOS that supports bifurcation; Gigabyte & ASrock tend to, ASUS rarely if ever does, other brands YMMV, but regardless you should check mobo manual before purchasing and see if the settings are there.