There are a couple of ways to look at this.
First, +1 to carrying a spare in case the first one malfunctions. It's the same reason it's a good idea to carry a spare magazine. If one bad thing can happen, (getting attacked,) another bad thing can happen. (Your weapon counld malfunction or get somehow unable to defend you.) Because the whole idea is preparing for the worst, and redundancy is a good thing if you are taking defense seriously, I will never mock someone for carrying backup.
However, keep a couple of things in mind. You must be 100% accountable for the placement and concealment and safety of your weapon at all times. If you think this is no big deal, try it with a single gun for a while. (In my state, it's not even required that I keep it concealed, and it's still a trick.) Now multiply this by two.
A lot of things affect the literal helpfulness of a second gun. Is it placed where you can get it, strong or weak hand, is it uncomplicated enough you can use it under the absolute worst of circumstances, does it use the same magazines, or at least the same ammo as your primary weapon, DO YOU TRAIN WITH IT JUST AS SERIOUSLY AS YOUR PRIMARY WEAPON?
For me, a pistol is something you use to fight your way back to a rifle. I THINK, a spare magazine, a good flashlight, a cell phone at least 40 hours of higher pistol training, and A RIFLE BEHIND THE SEAT are all more important than another pistol. IF YOU DON'T REALLY KNOW THE FIRST ONE, DO YOU REALLY NEED A SECOND ONE?