Linux

Regarding Wayland, looking at this it seems that it might be pretty straightforward but then for example sway seems to be defining seats a little bit different. So it might require some tinkering in the end, depending which compositor you decide to use.
Loginctl seems to be X/Wayland independent and useful in both.

From what I've found it seems that there are more questions on the internet regarding something not working well in multi-seat or needing a patch for that, than how to set the multi-seat up. Situations like this usually mean that setting this thing up is quite straightforward. It might also be a niche, though

You can run multiple X servers for a graphical multiseat setup. It's a lot of work to set it up and most of the information about it is out of date though.

It's kind of romantic. Thinking of all the hoops they had to jump through to get this intimate setup just right brings a tear to my eye.

I have seen this used in school situations with 3+ seats per PC.

that can add up when dealing with a dozen+ PCs.

For home use, not much a benefit, but I did have it setup years ago for the grandkids.

Not only for saving money, as when we upgrade the PC, I'm going to be able to sell off most of the components that get replaced, but space savings is also a big part of it. We're going to be buying an RV to live full time in soon, and the less space it takes up, the better.

Plus, we have used it in this configuration for almost two years now, and it has worked without issue (EasyAntiCheat doesn't play nice, but we don't play many games that use it anyway). And honestly at this point, I kind of enjoy the challenge lol

I mean, you save money by not having to buy another PC...

Not entirely sure why this reply is being panned (was at -6 when I first saw it).

OP is in the process of upgrading their PC to a Ryzen 9. If we make the assumption that this Ryzen 9 is on the AM5 platform, the CPU comes equipped with an IGPU, meaning the RTX 3060s are no longer needed by the bare metal. So, installing a stable, minimal point release OS as a base would minimize resource utilization on the hardware side. This could be something like Debian Bookworm or Proxmox VE with the no-subscription repo enabled. There's no need for the NVIDIA GPUs to be supported by the bare metal OS.

Once the base OS is installed, the VMs can be created, and the GPUs and peripherals can be passed through. This step effectively removes the devices from the host OS -- they don't show up in lsusb or lspci anymore -- and "gives" them to the VMs when they start. You get pretty close to native performance with setups of this nature, to the point that users have set up Windows 10/11 VMs in this way to play Cyberpunk 2077 on RTX 4090s with all the eye candy, including ray reconstruction.

Downsides:

• Three operating systems to maintain: bare metal, yours, and your partner's.
• Two sets of applications/games to maintain: yours and your partner's.
• May need to edit VM configs somewhat regularly to stay ahead of anti-cheat measures targeted at users of VMs.
• Performance is not identical to bare metal, but is pretty close.
• VM storage is isolated, so file sharing requires additional setup.

Upsides:

• If you don't know a lot about Linux, you'll know a bunch more when you're done with this.
• Once you get the setup ironed out, it won't need to change much going forward.
• Each VM's memory space is isolated, so applications won't "step on each other" -- that is, you can both run the same application or game simultaneously.
• Each user can run their own distro, or even their own OS if they wish. You can run Fedora and your partner can run Mint, or even Windows if they really, really want to. This includes Windows 11 as you can pass an emulated TPM through to meet the hardware requirements.
• Host OS can be managed via web interface (cockpit + cockpit-machines) or GUI application (virt-manager).

It's not exactly what OP is looking for, but it's definitely a valid approach to solving the problem.