Why would someone in the sniper's nest even attempt to contort their body and fire this alleged shot through a half open window?
Or: why would Oswald choose to shoot at a target moving across his field of view when:
1. he had already set up the boxes to support a much easier, and likely more accurate, shot with the target moving away from him;
2. he would have had one shot before the car passed under the tree and before it might speed off making additional shots less likely to succeed;
3. he would have to quickly do a change of position from standing and pointing the rifle down to kneeling with the gun pointing horizontally. This would increase the time interval before being able to make additional shots as the car proceeded down Elm and still within range. It would increase the likelihood of missing on subsequent rushed shots without being able to track the target before shooting.
If you buy into a shot coming at this point in time, it would be evidence of a 2nd shooter. "Almost on the horizontal" per "The Men Who Killed Kennedy".
Only if the second shot was not where the evidence says it occurred: a perceptible time after the midpoint between the first and third (last) shots.