That is a very accurate summary.
There were a few threads running netwide on this very topic but I never found a satisfactory answer on the question of what to do with a Mac Pro 2013 (with 2 (TWO) GPUs) and an ultra wide (49 inch) display. So I bought the display and started experimenting. Comparing output & input specs is really annoying, because you never know if a display port - without specifications - is 1.2 compatible UPwards. But for the MP 6,1 it was: Does MacPro 2013 Support Display port 1.2 - Apple Community.
Then I had to find something that would adapt the MP 6,1 MDP 1.2 output to a HDMI 2,0 input. The Orbsmart converter promised AND delivered. Of course I was afraid that I would end up with two screens in one, with this awkward black 'wall' in between them, but: no. Seamless.
So, coming back at your questions:
"you are connecting a much larger display than can be natively supported, but using TWO cables. Each cable carries the data for the left or right "half-display"."
Yes.
A much larger display can be divided into two separate surfaces, that both can be aligned to appear as one. I was unsure about this option, because I was used to a workspace where three HP 2409wg displays acted as one. But: no line in between.
"on the Mac, you use the "arrangement" pane to place the two half-displays back together in the correct right/left order."
Yes.
Under OSX Monterey I had to divide the wallpaper into three separate parts, but with LMDE6 it was a question of 'stretch' the background and 'go'. Arranging the half-displays in their correct order is (LMDE6) very easy.
"in the display, you have activated Picture-by-Picture feature (cousin of Picture-in-Picture) to get both halves displayed at the same time."
Yes.
With PBP you can assign screen halves to different input 'channels'. I was afraid both sides would be separated with this 'digital Berlin Wall' but this was not the case.
"... and your report is that they act without issue as ONE display, not as two fake displays side by side with a black line between them... and an additional feature is that the two halves can come in on completely different protocols, one HDMI and one DisplayPort family."
Yes.
ONE display, not two fake displays with this 'line' in between. And two different protocols, yes. In my case: left MDP/DP, right: HDMI 2.0. I was really wondering if this would be possible, but it is.
