Didn't you just say that JFK was hit by the first shot and JBC the second ?
When (what Z frame) do you have the first shot hitting JFK and what frame do you claim JBC is hit by the second ?
JBC was hit by the second shot in the back/armpit but the evidence is that a fragment from this bullet struck the windshield, windshield frame and sunvisor and another struck Tague, so it did not cause the left thigh wound which had the characteristics of being struck by the butt end of an intact bullet. So, that means he was also struck by another bullet. The evidence is very consistent that JFK was hit on the first shot. That bullet went to the left of JBC's midline and did not strike the car inside. What does that tell you?
To determine when the second shot occurred study the evidence. It was, according to Greer, just before he turned around and saw JBC falling back. According to Altgens it was after he took his z256 photo. According to Nellie it was after she stopped looking back at JFK. According to about 40 witnesses it was after the midpoint between shots 1 and 3. According to Hickey it occurred when JFK's hair on the right side flew up. You should be able to figure it out from that.