You must be careful that you take just one of each because taking more of either can have serious side effects. Taking an A without taking a B, or vice versa, can also be very serious, because they must be taken together in order to be effective. In summary, you must take exactly one of the A pills and one of the B pills at one time.
Therefore, you open up the A bottle, and you tap one A pill into your hand. You put that bottle aside and you open the B bottle. You do the same, but by mistake, two Bs fall into your hand with the A pill.
Now, here's the problem. You weren't watching your hand as the pills fell into it, so you can't tell the A pill apart from the two B pills. The pills look identical. They are the same size, same weight, same color, same everything, and they are not marked differently in any way. What are you going to do? You cannot tell which pill is which, and you cannot afford to throw them away and start over again.
How do you get your daily dose of exactly one A and exactly one B without wasting any of the pills?