According to your gun analogy, your saying that a flanking bonus should apply to a bow in the same way it applies to guns in real life. However, I would like to point out that if a gun is pointed me it still has the same chance to hit whether there is somebody holding a knife at me or not. Pretty much doesn't get any easier than point blank range in my opinion.
So what you're saying is you would completely ignore that the knife is there and try to dodge bullets? You'll get stabbed in the back. Conversely, you could totally ignore the gun to avoid getting stabbed, and you'll get shot in the back. That's the beauty of a flank; that panic when you don't know which threat to focus on, when your attention MUST be divided for any hope at avoiding both. That's a flanking bonus.