Makes sense to carry a New York reload.
It's been patented some what at the Steel Challange and other competitions for some events.
To be truthful there are variations of this. Get to that in a moment.
Worked well for the New York Stakeout Squad with Jim Cirillo and other highly trained and professional police officers from that outfit. Granted, things that work well for LEOs and Special units don't always work well for civilians, but this is one of those cases where it works no matter if LEO or civilian.
There are even holsters made for this kind of carry that work. Won't work for everybody under every condition, but will for some.
BigBadBull hit it on the head, Practice and proficiency are paramount for this if one uses this method, like any other method.
One carries two of the same or similar guns. Either the holster set carries two next to each other(one on each side), or one accesses the guns form seperate, but easy to get to holsters on the body.
Instead of reloading with speedloaders or magazines one literally drops the first gun to the ground and is accessing the second gun at about the same time.
This has been timed by the professionals(includes some well known shooting instructors,too) and practically all cases,depending on the user's abilities, the second gun was faster than a speed reload or a speed reloader reload.
This was done with stress added(safey rules were followed to the letter of the law) to make sure this could be done under real world conditions realistically. Not as just some stunt or new tacti-cool fad.
Most of the guns used were either small but potent guns like the Ruger SP101 and other snubbies, or smaller but potent pistols of about the same class and size. Medium sized pistols or revolvers this would still work with. For larger pistols this can still work, but there may be some problems of getting the egun out fast. A little experimentation could make using even large pistols work.
Down side was the first gun takes a beating and secondary ammo still was recommended to be carried to load the second. However if inexpensive guns are used one will not be worrying about the first gun if speed and accuracy are of the essence.
That is for those who care to carry this way.
There are limitations and variables to all this that make this not for everybody. For those who will practice and become professional level qualified, this may be a good choice. This whether LEO or civilian.
Second gun carry is recommended for LEOs and civilains. If the first gun goes belly up(happens enough to warrant this as cor consideration) then one is defenseless under msot circumstances. Yes, one might be carrying a knife, but a handgun lets one take care of a bad situation better.Especially during a gunfight. This may seem obvious, but the 'hey, I got a knife' thing may very well not work.Especially against multiple attackers or other Murphy variables. If it can go wrong, it will.
For others, myself included, staying with magazines, speedloaders, and SpeedStrips seems to work just fine.
There are plenty for who this would work for. Again, the level of training,practice, and staying at that high proficiency level are key before going whole hog on this.
A number of bowling pin shooters have used this method and describe it in articles, in books, or periodicals. Jim Cirillo has books out on this as well as Mas Ayoob.
I figure some of these threads on tactics or methods are being taken from Mas Ayoob articles or books. Too coincidental? Maybe.

;D

Many other trainers also figure into this.
