Just to root out the x-y factor, could you elaborate on your goals and how you wish to use it? I think "multi-seat" is a bit of a fuzzy term at this point and perhaps other solutions than what it's typically referring to is more appropriate for your use-case.
I haven't set up a single-GPU classic multiseat myself but a cursory reading from Debian wiki and Arch wiki Talk say it should be possible. There is mention of Xephyr, which QubesOS is using to do something similar with their "GUI domain". Maybe looking at that can be useful for you.
Two other things to consider could be VM with PCI-passthrough of the GPU (this does require a second GPU or an APU/iGPU in order to still have graphics on the host OS), or running secondary X servers with TigerVNC that you can then run fullscreen using vncviewer. The latter scales very well on a single GPU and works great for usual desktop/browser/dev but I'd be curious about gaming performance.
https://doc.qubes-os.org/en/latest/user/advanced-topics/gui-domain.html
https://doc.qubes-os.org/en/latest/developer/system/gui.html