Back in the days of the Old West, there were a lot of two-gun shooters. Movies notwithstanding, these old gunfighters seldom fired both guns at once, and it was certainly never done with any positive effect. Instead, that second sixgun was there in case the first one ran dry and the fight was still on. Going for another loaded single action was much faster than trying to reload the one in your hand.
Even in today's climate of autoloaders and double-action revolvers, carrying a second gun is not a bad idea. Some folks have coined this practice as a "New York Reload." Well, let me tell you, wherever you might reside, packing two guns is worth considering, and there are several reasons for this.
Should your primary handgun run dry in the middle of a fight, it might be a good idea to transition to that second gun. To be properly informed and able to make the right choice, it's a good idea to have someone time you while you do a speed reload and then time you while you transition to the second handgun. Which is faster?
Another reason to carry a second defensive handgun is the problem of stoppages. Of course we all practice malfunction drills and, if your stoppage is a failure to fire or a failure to feed, you can get back in the action fairly quickly. But, if your malfunction is something more complicated, you are going to be much faster getting that second gun into action.
Revolver shooters who experience a failure to fire simply pull the trigger again and cycle to another chamber, but if you're dealing with an ejector rod that has loosened and backed out too far or a high primer that has tied up the cylinder, transitioning to a second handgun makes a lot of sense and will be a good deal faster.
Finally, there are those cases—not all that rare—of the bad guy focusing his attention on the threat, in this case the gun in your hand, and shooting at it. When the bad guy's bullet disables your handgun, you will really appreciate the fact that you have another one on your person.
Some folks might make a big deal out of the fact you are carrying two defensive handguns. They might even accuse you of being paranoid. But what do you have to be paranoid about? You're wearing two handguns, aren't you?