Why do you think she would not have been able to see both JBC and JFK at the same time?. Not everyone suffers might from tunnel vision, John.
It has to do with what she would have been focusing on. You think she would have been more focused on which way JBC's shoulders were turned than on the POTUS raising his arms in front of him in reaction to having been shot through the throat. It's just another in a long list of goofy things you believe. Let's just list them.
At Z164, JBC starts to rotate to his right to look over his right shoulder in reaction to a gunshot that wouldn't be fired until on second later.
At Z193, Oswald fires a shot through the tree between him and JFK even though he only needed to wait one more second to have a clear shot.
JFK is shot in the back and the bullet exits from his throat. Even though JBC is sitting a half a body width to the left of JFK, the bullet miraculously misses to the left of JBC's torso and strikes JBC in his left thigh which for some strange reason is splayed way out to his left even though he is rotating to his right.
The bullet that exited JFK's throat without hitting any bone only has enough velocity to penetrate only an inch or two into JBC's thigh, even though a 6.5mm Carcano bullet is capable of penetrating 3 feet of pine wood.
Despite JFK having been shot through the throat and JBC having been shot in the thigh, JFK continues to calmly wave to the spectators to the right of Elm St. and JBC just as calmly continues with the rotation he began one second before he was shot.
33 frames after both men have been, both men finally react to being shot at exactly the same time when JFK raises both his arms up in front of his throat and JBC flips his right arm upward, a rather strange way to react to being shot in the thigh.
After flipping his right arm upward in reaction to being shot in the thigh, JBC twists and dips dramatically to his right in reaction to his thigh wound.
JBC continues his reaction to his thigh wound by turning his shoulders about 90 degrees so they are parallel to the side of the car and his head turns 180 degrees to the rear so that he is facing both JFK and the shooter.
With JBC's shoulders 90 degrees to the right at about Z270, a shooter from his right rear fires a shot into his back near his right armpit that miraculously exits from under his right nipple.
Oswald, after taking about 4 seconds between his first and second shots, fires the next shot in the bare minimum time of 2.3 seconds for an aimed shot and scores a perfect headshot.
Your scenario requires one fantastic occurence after another. Ten in total. Did I miss any?