Can I add pills?

So I take fluoxetine for depression and anxiety. I need to take 40mg a day. I have enough 40mg pills to get through the week, but I also have a bunch of 20mg pills. They are both delayed-release capsules. The difference is basically the dose. I can schedule a psychiatrist appointment to get a refill, but that requires a phone call... which means I'm going to put it off as long as possible. That's definitely what smart people do.

I know how math works, e.g. that 40mg = 20mg + 20mg = 2×20mg. I'm asking whether two 20mg pills will work like one 40mg pill. If not, would I get more or less medicine, and would I get it in the same time?

More generally, if you have X pills and you need to take kX (k a positive integer) pills, can I take X k times and expect the same result as taking a kX pill?