One of the most difficult techniques in karate is the yama kamae or manji kamae. The bunkai explanation for it is that it is a defense against multiple attackers (to the front and to the back). It is not as if we are a super hero, so why does such an explanation exist? I believe it is because either they are confusing movie action with reality, or they have never seen the original bunkai for this technique. It is hard enough to defend against an attacker from the front; I don’t believe it is possible to defend against an opponent to the rear who you can’t even see.
If you try to defend against two opponents simultaneously, you will see that you would have to train as if you were a stunt actor. If, let’s say one could defend against attacks from the front and the rear, and even manage to counter against the opponent in the front, the rear opponent is not going to wait for that attack to finish.
Also, even if one is attacked from the front and rear, one can avoid both attacks by moving. There is no need to increase the risk factor when one is already in a dangerous situation. Even beginners can understand this if given a rational explanation.