The "Two is one, one is none" mantra applies to guns. Basically, the idea is that for some reason, your main gun might not serve you (malfunction, it's taken from you, etc), and that you should have a back-up. I'm not going to comment on this mantra, as I don't even have the single gun (yet).
Although many of us carry 2 (or 3, or 4) knives, I doubt there's many who carry one as a "back-up". Those of us who carry multiple knives carry different styles of knives, for different purposes. I myself have 4 knives on my person at any given time, plus a multitool in my backpack. I have a main folder (plain edge), a secondary folder (either a Tasman Salt PE or Atlantic Salt SE) for work use (boxes, opening bags of greasy ingredients, etc), a SAK Cybertool 29 (particularly for the bit driver), and a LM Squirt S4 on my keychain. All these knives have different purposes, and there's little redundancy.
The blades on my SAK are a bit redundant, and I do keep them razor (literally) sharp, just "in case" for some reason I need them to be. Since I carry my SAK almost purely for its non-knife tools, I never have to use the blades, so I keep them ready for the most unlikely of contingencies (like an emergency tracheotomy or something).
If I were in law enforcement or emergency response, military, or expected to be in the woods or something, maybe I'd feel the need for a redundant back-up knife (probably a folder backing up a fixed blade), but for me, right now, that's not necessary.