Personally having 2 is more annoying than anything imo.
As stated, it allows having a backup. A good idea, but really shouldn't need a backup if your primary pistol works properly. Also allows you to having one out, use up the rounds, and switch pistols versus reloading (depending on mag design may be quicker). Akimbo works, but more so as a use one, then the other style shooting, plus reloading is a pain, and aside from looking like a movie, not the greatest thing.
Down side is obvious weight, reloading 2 instead of 1 is annoying and time consuming, more tech work, plus, having to own and maintain a second pistol. This can be very costly.
Plus, if you get 2, you'll need a second holster of course....Wait, on second thought, get 2. :P