In addition to the notion of a BUG, the big reason why I carry two is to balance my clothing out for both comfort and concealment. Whether its shorts or pants with cargo pockets or a jacket, there's one on the right side and one on the left, avoiding tell-tale drooping of clothing. My match-ups are either my S&W 649 .38 and my PPKS in .380, or my Beretta Bobcat .22 with my Kel-tec .32.
In the winter, when you can get away with wearing a jacket in Arizona, I will sometimes have a more serious weapon concealed on the belt, namely my Colt Officer's model, but will still carry the other two.