So we have full control inputs in a high altitude flight regime where the rule is to fly with a light touch. Had a conventional linked control column been fitted to the aircraft, the PNF would certainly have noticed the gross control movements and probably been prompted to intervene at a much earlier stage. A classic case of the left hand not knowing what the right hand is doing and vice versa. Sure, the crew weren't communicating with each other in identifying the problem, but the situation was greatly compounded by a control system and instrument layout that effectively hid each pilots' actions from the other. The cockpit design and control layout are not designed for instinctive responses in abnormal flight upsets. Flight control inputs can be summed or subtracted if both pilots grab the sidestick controllers at once without one of them pressing an override button. Either pilot can press the button without the other being aware in a high stress situation so you end up with an aircraft that has unpredictable responses and a crew that are fighting each other (and the aircraft) at the controls without being aware of the situation, which seems to be the case here for at least part of the descent.