The way this works is: when you run out of your primary X and pull your backup X out of the closet, you immediately add X to the shopping list you carry with you. Then when you're in a shop, you simply buy things that are on the list and put them in the backup closet.

This requires developing two habits: 1) adding a thing to the shopping list when you fetch the backup item, and 2) actually using a shopping list.

Now I sympathize with the fact that different people have problems with different habits, so if it really doesn't work for you, then I guess you need something else :). But the benefit of this trick (or "life hack") is that it's free to use, doesn't require any purchase to set up, and doesn't tie you to any vendor.