Learning to take "double taps" starts with two aimed, controlled shots ("controlled pairs"), usually about 2 seconds apart. After the first shot, wait until the sights level out and the center of the target is once again where it should be, then carefully press the trigger for the second shot. I know, it sounds like you're not practicing what you want to do (your goal is two *fast* shots to COM, right?). But you *are* practicing what *needs* to happen- sight alignment, sight picture and trigger control (along with recoil management). You will find that it doesn't take too many repetitions before the sights start coming back to alignment and the second shot can be pressed off in about 1/2 of the time. Work at that speed for a few repetitions, then try to push yourself a little faster. When your 2 shot groups start opening up to 8-10", start over from the beginning ("controlled pairs"). You are now working on the balance between speed & accuracy (and gaining a lot of skill as you do). Recoil management and flinching are the major obstacles when it comes to putting two fast shots inside of an area that is acceptable for defensive situations. With a little practice, the second shot can be .10 to .15 seconds after the first shot.
Head shots require the shooter to slow down just a tad, due to the smaller area that the shot must hit. Hard focus on the front sight will get the bullet to hit the target where you want it to. We illustrate this point by having students shoot a 6" grey steel plate from 30 feet distance (it's *easy*). Then we put them on an 8" plate that is painted safety orange at 25 feet. Most students miss that big orange plate, sometimes repeatedly & to great frustration because it draws their focus away from the front sight to the target.