I would rather carry a knife than a second handgun or a second magazine, and I do. There's a number of reasons why. I have 15 rounds in my carry gun which I think is probably 2-3x more than I'd ever realistically need in a self defense scenario, or, if at home, at least would buy me time to get to my AR15.
As for a second gun, I think falling back to a knife is more realistic. If your primary handgun has malfunctioned there's good odds in that scenario that your attacker will now have closed the distance and be striking or stabbing you, and at such close range it's going to be harder to use a handgun than a knife. If your attacker is shooting at you odds are you will be dead before getting a second gun into action anyway. So I am not sure how many times a second gun would really save you, but a knife might.
Secondly, a knife is genuinely useful on a regular basis. The most common use for my knife is honestly cutting open packages and then cutting up the boxes for recycling, but there's lots of other things it comes in handy for. I use it half a dozen times for different things during the day, so I have come to love having it on me.
I personally put the knife on my belt after watching a video from my own home security system. I'd gone out to collect the mail in the middle of winter when it was getting pitch black dark by 5pm. In the security camera video there's a cougar that walks along behind me, and then wanders off. I had no idea it was there, and fortunately, it viewed me as a curiosity rather than a meal, and left me alone. However, had it decided to attack me, I'd never have seen it coming. Really very little opportunity to draw a gun and fire at a cougar that is already on you trying to bite your neck. Assuming it hasn't succeeded in biting my neck yet and killing me, if I'm able to fight back at all I figure my best odds of survival at that point would be to pull out my knife and stab it.
Plus my knife is really light and easy to carry compared to a whole second gun. The knife is attached horizontally on the front of my belt so I can draw it with my right hand. I really don't even feel it's there, whereas when I strap on a gun--I know.