I often carry two everything:TWO handguns, TWO spare mags, and TWO knives! Kinda makes me like Noah, doesn't it. BTW, I live in Kentucky, which to my knowledge has no restrictions on what kind of weapons or how many can be carried.
Do you plan on carrying a BUG as a civilian? I do not even know any retired LEO that carry more than one gun once they can retreat and no longer have to run towards the sound of the gunfire. I thought about it myself when I first carried and realized that my fear level was not that high but then again I was never the fearful type. However I do prefer snub nose revolvers and on occassion will carry an LCP instead of a speed loader as they pretty much weight the same. :) When it comes to semi autos I only buy high end guns from top manufacturers and consider a spare magazine as part of the gun and always have one with me.
From talking to guys at the range and gun clubs it appears that many do not carry spare mags and carry guns are low enough on the reliability scale to make them want to carry a BUG. I also can understand the concern about losing your gun to an attacker and having a backup but short barreled guns are hard to take away and a good knife can make anyone let go of your gun. I would rather take a weapons retention class than lug around a second gun. I barely can put up with carrying one. :) However since what anyone else carries does not affect me you have my permission to carry as many as you want. :)
P.S. if you carry two pennies and flip one that comes up heads, it does not change the odds of the second one coming up tails. If it did, roulette would be easy to beat. My point is that a second gun has the same chance of failing as the first so why not carry 3 or 4 or more? See, math can be fun. :)
There is no magic number of rounds or guns to carry. Much simpler to make sure you buy and throughly test out your carry guns with your carry ammo (and not just with one box or less as many do) so the odds of them failing are small. Or carry a revolver like I do most of the time especially after having a once reliable semi auto fail on me during a reliability check at the range the week before. :)