Kevin Baker laments his lack of speed in firing "controlled doubles" at a recent shooting match he attended. In his post, Kevin links to a classic Chris Byrne post that thoroughly explains and illustrates precisely the what and why of the issue. I should also note that I've met Chris and seen him shoot (at the NoR Fest/Conf last year); not only does he clearly understand the difference between a controlled double and a doubletap, he can demonstrate same with depressing regularity and accuracy.
With a brand new, straight-out-of-the-box, never before fired by him 10 mm no less.
All that said, I'm going to argue the position that, outside the bounds of formal competitions like IDPA/IPSC and the like, the standards of performance Chris and Kevin seek to achieve are not entirely realistic for actual pistol gunfights (says the man who is loudly grateful never to have actually had to engage in one).
Hold your hand in front of you, palm toward you, fingers extended and touching each other. Then make a fist with the thumb resting outside the closed fingers. I should point out that attempting to punch someone with your fist in this configuration will almost certainly result in more damage to your fist then to another's face; also, the area demarcated by your fist (base of the palm to the closed fingers x the thumb knuckle joint to the blade-edge of the palm) is a reasonably accurate simulacrum of the size of your heart. Place your closed fist over the mid-point of your chest and you will have a clear illustration of the target area involved. Now, center the last joint of your little finger on your upper lip and take note of the portion of your face your fist covers.
As part of your next trip to the range, make this same fist but this time extend your arm towards the target you just shot. If the X-round (6, 7, 8, 17?) string you just shot is covered by your extended fist, I'm going to suggest that you did indeed "got him" and that that's more than good enough.
Which is the point underlying this post; just how much and, at least as critically, how fast is "good enough" when your target is another human trying to shoot you and not just a steel plate or piece of paper?
My belief is that one shot "within the fist" on a consistent basis is of greater importance then almost any number of shots placed adjacent to the critical location. Keep in mind the dimensions of an "A" shot as described in Chris' post. The area of my own extended fist is ~ 2.5 inches wide by 4 inches high, considerably smaller then the 6" x 10" area permitted in competition. In a defensive handgun fight, a potentially lethal wound will almost certainly limit the number of return shots you will have time to deliver - which is still really bad news for your opponent.
Especially if we reverse the scenario and you are the opponent.
I contend that, while the averaging effect achieved in IDPA-type competition is an excellent test of a shooter's overall skills, the value of a single well-placed first shot is of overriding importance. Furthermore, if you are capable of consistently placing a shot within the fist on a human chest, you are equally capable of doing so to a human head. Should that be the case (talk about your critical self-analysis!), then for the purposes of a defensive gun fight, if both target areas are equally clear* the head shot ought to be the primary choice. Remember, this is handguns, not rifles, and at a probable distance between shooters of 7 to 10 yards maximum (anything much beyond that range starts to call into question the whole "defensive" aspect of the thing, don'tcha agree? You're gonna get sued anyway, why make it easy for them?).
I choose to carry a 1911 pattern .45 acp semi-auto as my primary weapon, with a S&W 431 PD in .32 H&R Magnum as a pocket/back-up weapon. If you also choose to fire a bullet of lessor mass than a .45/357/44 mag (9 mm or .38 for example) you too will be more likely to find a second shot necessary. I do agree that delivering a follow-up round to the same fist-size area as the first shot within one second of firing that first shot is a realistic objective. You aren't likely to be given more time then that by the other shooter in any case, so the impetus behind Kevin's desire isn't simply confined to seeking competitive advantage in a formal match setting. Now that he too is (will be soon?) carrying concealed, the ramifications of doing so need to be considered as well.
Shooting competitions, practical or otherwise, are structured to emphasise the competitive relationship between the shooters pretty much as a requirement of their design, whatever the practical/defensive intent might also be. If you're going to carry a gun off the range as well as on, I believe you should regularly train to use your weapon to an off-range requirement on a regular basis, in addition to training for any other shooting setting that attracts your interest. Just keep clearly in mind the differing circumstances of your varied interests, that's all, and be careful not to practice one to the exclusion of some other.
*A "clear" target is one that is unobstructed, both in front and behind, along the likely trajectory of the fired bullet.