I am going to be comparing a S95D 55" QDOLED vs a Hisense U8N 55" MiniLED. Windows HDR usually looks better with most games and I always have it on for any game I play but I don't think it is possible to have it on, even if you had a GPU with 2 HDMI 2.1 ports so I just wanna confirm this is the case. The reason why it doesn't work is because HDR takes into account the luminescence values for each screen so if they are different, even by a small amount then it can't be duplicated, SDR doesn't care about that.

This is the case with my Display port to HDMI adapter that I have plugged into my White AsRock Taichi 7900XTX comparing my older 55" Samsung QN90B vs my 55" Hisense U8K, HDR won't work at all in duplicate monitor mode, it makes me wonder if I had a dual HDMI 2.1 port GPU , if it would let me have HDR On but I doubt it will.

Can anyone please confirm this?

Dual Monitor modes are best for comparing TV's/Monitors to see is they look equal or if one looks minor to moderately to significantly better than the other, I guess they can only be compared in SDR mode but I suppose it can be assumed if one looks better in SDR then it will also look better with Windows HDR On.