While I am hardly a devotee of pistol play, I have used two pistols on occasion before and noted several advantages and disadvantages. It is great because you can keep one holstered for when you need to reload or, if you want some extra firepower, shoot akimbo.
The drawback for me was that it was a little bulky and harder to reload if you had both drawn. However, I was using two Ariakon ACP 2.0s with two spare magazines, so I certainly could have cut the weight/bulk down, especially with newer pistols. I also really dislike thigh holster rigs and I had one on each leg.
I would personally run two pistols if I was ever to go into anything other than a small rec game. I would think that with the newer mag fed designs with quicker reloading and still throw out a serious amount of paint with a single pistol. That being said, I am sure AgentSmith will see the pistol signal in the sky and come running to this thread and give some solid information based on true pistol experience.