Yes, I know all that. Except it doesn't work. At all.

Ten years ago there were cursor clipping issues, cursor coordinates issues and crashes and I've been home-baking patches for that.

Also it was impossible for one X session to span across two GPUs. Dunno if that was improved.

Now it's bit better, but for sure your amdgpu will entertain you with little nice crashes when you run something heavy on a scaled display.

I'm not even talking about VRR, HDR and all that stuff.

In that time I've had Hidpi work perfectly on first on Nvidia then recently on AMD GPUs on several different distros and desktops all running on X on several distros. They all worked out of the box and were able to scale correctly once configured.

The totality of my education on the topic was reading the arch wiki on hidpi once.

AFAIK one cannot span one x session across multiple GPUs although AMD had something that it once referred to as "eyefinity" for achieving this.

It is rarely needed discreet GPU often support 3 or even 4 outputs

One may wonder if you tried this a very long time ago back when AMD sucked and Nvidia worked well in 2005-2015