Bullets travel slow enough that we can use the formula E=1/2mv^2 to calculate. The rifle that Oswald used had a muzzle velocity of 700 meters per second, and used 128 grain ammunition. Plugging that into the KE formula we get E=(1/2)(0.00829 kg)(700 m/s). When we calculate that, the total Kinetic energy of 2.9015 joules. For comparison, 1 joule is equivalent to 0.000239 kilocalories, the unit of measure used to measure food.
You also seem to be forgetting about Newton's 3rd Law of Motion. Even if there was a 2nd gunman who shot Kennedy with enough force to visibly knock his head back, that same amount of energy would have also been exerted on the shoot as well. Meaning that your second gunman would have had a broken collarbone, and several ribs, if he had used a rifle roferful enough to exert enough energy to knock Kennedy back and to the left.