Principle of Two Weaknesses

Imagine you have been assigned to make a penalty shoot-out for your team. Your eyes are focused on the right side from the goalkeeper. You are determined to shoot there. Obviously, the goalkeeper prepares to jump into that direction. You gradually approach the ball, you shoot, the goalkeeper jumps to his right, whereas you directed the ball to the... left side you the goalkeeper!

Or how about an analogy with warfare? How many battles have been won in the following fashion: one battling side deliberately exposed its position and provoked the opposing army to attack. The opposing army threw all forces into that attack, and then the defending side revealed the hidden part of its army that counterattacked the enemy from behind. Being attacked from both sides, the opposing army was destroyed. 

Both examples are overly simplified. However, I hope that they do illustrate my point. Same in chess! Creating one weakness is usually not enough. Your opponent will divert all his resources to its defense. However, if you create second, or even better, third weaknesses - then your opponent will not know from what side you will attack. Then, you will combine threats between the weaknesses, worsen your opponent's coordination, and then deliver the final blow!