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