I don’t dispute that they are reacting at the same time to the same shot. I just disagree that the evidence supports it being the second shot.
One of the three shots missed the target. We know that both men were struck where the film shows them being struck. Energing from behind the stemmons sign, and the fatal shot.
This leaves only one other shot, and that shot had to have been fired early, like around Zapruder frame 155/156.
A. Zapruder's camera jiggle proves that a shot was fired there.
B. The sudden head turns of both JFK and Governor Connally-both of whom were looking to their left. And one thing most people do not notice is that Connally was looking to his right
when Zapruder first began filming, turned to his left, then
suddenlly jerked his head back to the right, where his head remained turned during the entire travel down Elm Street until he comes out from behind the sign, which is the first motion he made-other than taking his right hand off the top of the side rail of the car.
This is exactly what the Zapruder film shows. Three sudden reactions to three rifle shot sounds.