On occasion I will carry two revolvers. Generally, I use a 442 with another jframe or lcr or m85 in separate pocket holsters (one left and one right) since they all work and feel about the same. But, for the most part I just use a single jframe and a speedstrip and/or a speedloader.
Also, I thought this guy has a pretty good video on this subject
Back up Guns for Concealed Carry. - YouTube
I finally got around to watching this video. There are a couple of points that he doesn't address in favor of carrying a back-up gun.
1. Offhand-side access. He demonstrates drawing his primary gun from his right side holster using his left hand. This is a valid technique. I practice it both going around the front and around the back. I use a clip IWB holster and while some people don't like them because they can shift, that ability to move does help when performing an offhand draw.
However, there is one problem with this approach: Not everybody has the mobility/flexibility to perform this draw. While I can perform this draw, I'll admit some days are more difficult than others due to soreness, joint stiffness, etc. Others may be even more restricted, whether it's due to injury, arthritis, etc. In situations like this, having a BUG available to the offhand can be beneficial.
2. This is the single biggest omission as far as I'm concerned. One of the most important reasons for carrying a BUG, and the reason that most interests me, is in the event one's primary gun doesn't work. Granted, a well-maintained, good quality gun, loaded with good quality ammo, is highly unlikely to malfunction under most circumstances. But it is a possibility, and one's options are to clear the malfunction or draw a BUG. I would've liked to have seen him do a comparison between clearing a malfunction and drawing a BUG.
Revolvers are generally very reliable (as well as semi-autos), but they can malfunction. While there are malfunction clearing drills for the revolver, a better solution would be to draw a BUG. Even with a semi-auto, it may be quicker to draw a BUG than attempt to clear a malfunction, especially something like a double-feed. Plus, there are some malfunctions that can't be cleared. For example, my first 642-1 broke it's hammer stud. I'm glad I discovered it while dry firing instead of during a life-threatening situation.
Does that mean everyone who carries a gun should carry two (or more)? That's for each person to decide, based on their own situation. But I don't think it's something that should be dismissed without consideration.
And if a police officer or prosecutor decides to make an issue out of someone legally carrying more than one gun, it shouldn't be too difficult to find instances where a BUG helped someone survive a violent encounter. Massad Ayoob and his Ayoob Files would likely be a good resource on this matter. If necessary, one's attorney could probably subpoena someone from the police department, such as a firearms instructor, and find out what their policy is regarding BUGs. Not to mention instructors who advocate, or at least recommend, carrying a BUG, even for non-LE.
Final point: While I'm not an expert on the carry laws of all 50 states, the only one I'm aware of that specifically prohibits carrying more than one gun concealed is New Mexico. As always, check your local and state laws to make sure before carrying multiple guns.