Deep in the mysteries of the body, the ovaries are developing eggs. At any given time both ovaries are developing ten to fifteen eggs. The ripest egg is then released into the fallopian tubes at about the same time within the woman's cycle each and every month. After this egg is released it will survive in the fallopian tubes for about 24 hours. It then dies and passes into the uterus, unless it meets a sperm and is fertilized.
Contrary to common belief the ovaries don't "take turns". Which one releases an egg is random and one may release an egg several times in a row before the other one "takes a turn". On occasion one ovary will release an egg and the other one will release an egg sometime with in the next 24 to 48 hours. This doesn't happen often, but it does happen. Usually if an egg becomes fertilized the body will start to send hormones through the blood stream. This prepares the body for the fertilized egg to implant in the wall of the uterus. This usually stops the ovaries from releasing eggs. If the hormones don't increase quickly a second egg may be released. Because sperm can live in the reproductive track for seven days it is possible to have this second egg fertilized easily and with out any additional sperm.
Some women can feel their ovulation throw pain and discomfort. These women often know when they ovulate (with in about 48 hours) and they can even tell you which ovary was responsible. Often they panic when they feel ovulation occur from both ovaries, however, it isn't unnatural and it doesn't mean that anything is wrong.
When both ovaries ovulate and both eggs meet sperm in the fallopian tubes and get fertilized fraternal twins are created. These are twins that aren't identical and are only related as much as brothers and or sisters can be related. These twins are the only type that will ever be boy and girl. Additionally you can have twins that look nothing a like and are different in many ways. This happens mostly from both ovaries ovulating at or near the same time. It is also possible for one ovary to ovulate twice in a row, but even less common.
Others, who don't feel ovulation pain and don't get pregnant with fraternal twins will never know if they have ovulated from both ovaries. However, they could be doing it often!