Mello2u - I was always of the opinion that the reasons for double shots were 1) during the stress of the incident and movement/action occurring, accuracy may not be perfect. 2) an aggressor may have adrenalin, drug or other driving forces that may limit the effect of a first shot (that could be true even of a rifle).
I did see you specified "defensive handgun cartridges". I was wondering if you are referring to those that are lower velocity to avoid over penetration (but maintain expansion capabilities)? If so, I have wondered how those compare to a good HP myself.
Two shots or three shots are taught for those reasons you mentioned above; as well as, the reality that the handgun cartridges I listed just won't work to stop an aggressive threat unless you put two or three well placed center of mass bullets in that threat (in a significant percentage). That part of the reason why there has been so much discussion over one-shot stops. Whether "the stop" occurred from true incapacitation or due to the threat believing that they should have given up merely because they were hit, or shot at and not even hit; is part of the problem in trying to determine what shootings to include in data samples.
I specified "defensive handgun cartridges" as a personal choice of cartridges I feel I can shoot competently in a defensive mode. This is mostly the real of control. I also feel they are in the range of power that I would be willing to use. That use is assuming a good hollow point bullet design that functions in my handgun/s.
Penetration is "a whole nother" discussion.
NRA Life Member - Orange Gunsite Member - NRA Certified Pistol Instructor
"When plunder becomes a way of life for a group of men living together in society,
they create for themselves in the course of time a legal system that authorizes it and a moral code that justifies it." Frederic Bastiat