One of my reasons for carrying the second weapon is religious fanaticism.
My “conversion” happened one morning, in 1988 or 1989, when I arrived at home, after working a duty shift of police patrol, and discovered that there was a rattling/tinkling sound, within my S&W Model 58. I removed the grip panels, and two parts of the main spring fell onto the table. (S&W K/L/N-Frame revolvers use a leaf-type main spring.) Well, OK, this is more like Obsessive-Compulsive, rather than religious, and not so Obsessive-Compulsive that I always carry two firearms, but, yes, I am all too aware that a firearm can fail, due to parts breakage.
Notably, I not not find any evidence that this main spring had been “shaved,” to reduce trigger pull weight. Manufacture of the individual revolver had been about 1974 or 1975, when S&W made a run of these for San Antonio PD. I had bought this Model 58 in 1985.
Another reason that I have carried more than one firearm, when not in the line of duty, is because of I have much affection for the Ruger SP101, which is a five-shot weapon, and, like its J-Frame cousins, does not have much “work space,” for a speed loaders, when the cylinder is swung to its loading position. Unlike the J-Frame, the factory grip of an SP101 fills my hand nicely, I can get all of my skinny fingers onto the grip, and the “heel” of the grip reaches all the way to make firm contact with the “heel bone” of my hand, an added point of contact. This additional point of contact, and the resulting stability, has always meant better accuracy potential and shot-to-shot recovery. Then, when arthritis caught up with me, many compact handguns became intolerable to fire enough, with my less-healthy right hand, to get a good training effect. So, the SP101 is my default “compact” handgun, with a grip that enables it to handle more like a duty weapon, at least in my hands.
Why bother with low-capacity revolvers? Well, for one thing, my right hand is getting gimpy. There is more going on, than just arthritis. I only trust my right hand to provide a stable platform, for reliable cycling, with a very few select duty-sized auto-pistols. Some amount of effort is required to dress around these pistols. A revolving pistol does not mind if my Ulnar Nerve is having a bad day, and concealing a spur-less hammer revolving pistol is a comparatively simpler task.
Notably, it is NOT unusual for me to dress-around a Glock G17. 1911 pistols, too, occasionally. I have been dressing around full-sized duty handguns since ~1985.
My back has not normally bothered me, but, twice, in my life, I tweaked my lower back in such a way that the muscles would fire-off most painfully, if I carried the weight of a firearm, unless I carried a balanced load of two firearms, one in each front pocket. Each time, having 2+ compact revolving pistols, and 2+ pocket holsters, enabled me to be armed well enough for most foreseeable problems.
A related personal factor in carrying two weapons is to maximize the benefit of being relatively ambidextrous. I am not fully ambidextrous, with all tools and tasks, but I am, with most revolvers, and some number of autos. Notably, I write lefty, and throw right-handed. Long ago, I chose to carry on my right side, because the draw felt more natural when done right-handed, especially when using then-mandated L-Frame revolvers, from the then-mandated low-slung duty holsters, with the way that the draw was taught, in those days. (Texas did not have generally-legal handgun carry, at the time, so, I could not carry a handgun until 1984, when sworn-in as an LEO.) I may well carry lefty, either a large or compact weapon, but, with so many draws, over the decades, from my right hip area, I still feel most comfortable, and most comforted, if there is at least an SP101 at ~0300. Sometimes, while LEO-ing, I would draw, more than once during a shift of duty. Though now retired for 6+ years, old habits, hard-wired into the brain, die hard.
This is my way, not THE way.