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.