A Controlled Pair is when the shooter is engaging a threat and fires two rounds in rapid succession. The second shot in the pair, is fired as fast as one is able to acquire a clean sight picture and align their sight(s).   This Controlled Pair method came about after the "double tap" technique. The problem with the double tap was that the second shot in the pair was fired off before a second accurate sight picture was acquired. You'd get one shot in the thoracic cavity, and another in the lower gut for example. Hence the controlled pair was ruled king in many circles.

Here is the problem. Training only a Controlled Pair response does not solve the problem of a non standard response. What if 2 rounds do not stop the threat? If every up drill you do on your flat range work consists of 1 or 2 rounds, you are ingraining a 2 round response when its show time. While you spray 2 rounds at a dude, pull your gun back to the compressed ready or drop to low ready with a carbine, because "the drill is over," that threat may be sending rounds into your face because those 2 rounds didn't stop him.