I don't currently do so as the guns/carry methods I use make carrying two guns a bit burdensome when combined with a reload, cell phone, wallet, keys and (sometimes) flashlight. I would have to make some adjustments. But is there any reason
not to? These sound like some good reasons:
1. It's faster to draw a second gun than to reload, should you shoot the first one dry.
2. It's also faster than clearing a malfunction.
(Note: The above do present a problem in what to do with the first gun.)
3. With the extremely close distances found in many gunfights, you could be grappled and have your gun taken away, dropped, etc.
4. In a reverse role, suppose you feel threatened but the situation doesn't
yet call for drawing your gun, which is an obvious and overt act and could be considered "brandishing." But you can unobtrusively slip your hand into a pants or coat pocket and already have your hand on
a gun.
5. While not precisely on topic, you do need a
replacement gun if you're in a shooting. The police are going to take it away from you and hold it until the investigation is complete and/or the case adjudicated.
I would consider
three guns to be crossing over the line from prepared to paranoid.