This is very, very speculative at best . . .
Maybe their procedure was to swap legs, and the pilot doing the flying did so in the left seat, but the Captain was still in charge.
In my experience one guy flies, and one guy talks, and it seems to work reasonably well. In the military we sometimes changed seats, but not in the airlines.
I seem to recall that the guy doing the flying could always reject the takeoff, although I’m certain this got exciting at times.
(I flew for PAA for years and the first thing any F/O checked out on a flight was the mood of the Captain.)
If in doubt, reject, and we’ll discuss it later.
Both pilots had plenty of time, type ratings for the airplane, and ATP licenses, so they were working at it.
Also, they had recently passed sim checks, so they were up to speed on the unusual stuff.
The lower time pilot was doing the flying from the left seat – I assume.
At V1 everything is fine, at Vr it doesn’t, and when the nose doesn’t come up, you would say “what the H” because this is not something you have ever experienced, or heard of, before.
You have possibly eaten up 4500’ of runway before the other pilot agrees there is a problem and the reject starts. Now you are at the 6000’ feet of runway and shortly realize you are not going to stop on the runway.
We are so primed for a fire or an engine failure, maybe a runaway stabilizer control problem, but I do not recall a control not working at all.
Ailerons rigged to work backwards maybe, but elevators not working at all, is a new one.
Gulfstream builds a great airplane with quite a reputation so we will soon know what happened.
And I think the new airplanes with the fly by wire stuff might lead to more of this, so know where the breakers are . ..