As my previous reply was so brief, I wanted to add that one big reason I carry two is that I want access options, and am largely ambidextrous, anyway. (I am more of a lefty than "rightie," yet carry "primary" on the right hip.) If attacked on one side, I can throw up a block or guard with the arm/hand on that side, and access a weapon with the opposite hand.
A lesser reason, but still important: I can spread the considerable weight of two SP101 snubbies, or an Al-alloy-frame SIG P229 and an SP101, across a wider area, for greater comfort than carrying one larger steel weapon at one point on the hip.
I notice plenty of folks say they carry a knife on the side opposite their one gun, and there is nothing wrong with that; I think it is a good idea. I do it myself. But, having actually trained to deploy a knife in force-on-force conditions, I am more comfortable with the idea of pulling a gun with the "off-side" hand than trying to deploy and open a folder. One exercise during one seminar showed us just how likely we are to flub even well-practiced knife presentation when placed under some stressful pressure.
In the past, I carried a stout fixed-blade knife knife as secondary to the handgun, on the side opposite the handgun. I do consider a good fixed-blade knife to be the full equal of a handgun at touching distance, if the user is well-practiced. When I have more time to train, and locate another good training partner, I may well go back to carrying a fixed blade rather than the second gun. (Yes, legal in my area.)
