The rule of two is that two is one — and one is none. This is applicable to so many things in your life. As a starting point, I often like to think of the rule of two with things that you have around the house. So, for example, if you have one roll of toilet paper, you really don't have any toilet paper. Because when that one runs out, you're in trouble. So you really need two rolls of toilet paper at all time. It's a redundancy rule, basically. It's where this comes from.
This is one of my little pieces of advice for trying to run a life very smoothly. It's that, everything that you can possibly have two of, you should have it. Two shampoo bottles, two bottles of vitamins, two boxes of cereal, two cartons of eggs. You want duplicates of everything. And then, when you're down to one of those things, that's the sign that you need to buy the next one. In this way, you are never out, you're never out of anything.
It's applicable to everything in your whole life, everything that's important.
— CGP Grey
This is so spot on. I try to function like that, but I never put words onto this behavior. Here it is.
Think this way with computer files: you have only one copy of that file, guess what? You have no copy of this file. I even think it's applicable to work. If you have one source of income, in many ways, it's like you have no source of income. Because if something happens with your main job, you are in lots and lots of trouble. One source of income, no source of income.