All this is, of course, assuming that the father is the same. However, there have been more than one instance when the assumed father of the twins turns out to be the father of only one child. The scientific terminology for such an event is heteroparental superfecundation (HS).
How rare is a pregnancy?
Before we get into how HS occurs, a quick biology class.
During sexual intercourse, the human male deposits millions of sperm in the female reproductive tract. Inside the harsh environment of the female vagina, most sperm perish while whipping their tails to move up the female reproductive tract.
With the energy supplied by the mitochondria inside them, the sperm navigate the arduous path of the cervix and uterus and then up one of the fallopian tubes with the hope that they will meet the egg. Only a few hundred or perhaps even lesser reach this point.
To fertilize the egg, sperm needs to beat the odds of being in the correct fallopian tube, where the ovulated egg will be available. Further, it must defeat other sperm who have also beaten the previous odds and are now fighting to fertilize the egg. An egg is usually available for a brief period of 12-24 hours, so the sperm must be present at the correct time for pregnancy.