I did this and did that, trying to figure out how to have twins or triplets without leaving it to chance and I finally figured it out. It works for me, and hopefully it will for you as well.

TWINS: Use the fragrance on the man and woman and give one of them the baby boost. I have had twins every single time, even if they didn't have a baby the first time. I would try again when it would let me without using anything else and, yep, twins. I have had many sets of twins this way.

TRIPLETS: Make sure a twin takes over the house when their parents die. When you want them to have a baby use the fragrance on the twin, but not their spouse. Use the baby boost on the spouse. I have only tried 2 times, but both times I had triplets! I tried with both fragrances once, but it didn't work at all.

HOPE THIS IS HELPFUL!