As with all Airbus A/C, the two side-sticks are not linked (meaning moving one will not move the other), so the A/C instead uses a system of "side-stick priority" to determine which input to act upon in the event of simultaneous stick inputs.
So, assuming the captain had side-stick priority, then any inputs by the co-pilot would have been ignored, as the jammed camera was making a constant input on the captain's side-stick. If the co-pilot had pushed his own side-stick priority button, then he would have been able to pull the A/C out of the dive, allowing the captain to retract his seat and cease the input on his side-stick.
I know it's easy to say from an armchair, but I'm amazed that neither the captain nor co-pilot apparently thought to do that.
