if both pilots were pulling back on their sticks, wouldn't the captain have realised the problem with the wedged camera?
No. The control columns are not linked, they move independently. As I understand it, the Captain could not move his control column, he surmised (without checking on the FMA) that the autopilot wouldn't let him (I have no idea why he would think that) he tried to disconnect an already disconnected autopilot using the button on the control column. If he had held this for 45 seconds he would have disconnected the other Pilot's control column. Luckily he didn't. The other pilot pulled back on the cc, the aircraft summed the inputs. This had no effect since the protections had already kicked in to start to pitch the aircraft up in order to control the speed. Had the obstruction remained in place there are 2 scenarios, either the other pilot continues to pull back, the aircraft sums the inputs and maintains 1g flight in whatever attitude it finds itself or the other pilot disenables the Captain's cc by taking control. Luckily, in +g flight the obstruction cleared itself.