I am 33 and i have though about the problem. The problem is not that they can knock out each other at the same time. The problem is the simplified states we are using. We are implicitly assuming that a character can atleast be conscious or unconscious with the latter state limiting the actions of the charcter.
What instead if our game modeled the values of the arm. Like length, weights, speed, acceleration, mass and direction. Thus when the gameengine detects a collision between the arm and the character b head code to transfer the Force begins to execute. And thus the effect this will have on character b, eventually leading to falling down motion/animation. But in the same time charcters b arm will still be in motion since it is modelled as real world physic and it does not matter if character b in unconscious or conscious since the arm will have its own (relativly to the body ofcourse) momentum.