Although not very common, fraternal twins can not only be conceived at two different times (i.e. a day or two apart), but they can also have different fathers. This happens when a woman who has ovulated with multiple eggs has sexual intercourse with one man and conceives and then has sex with another man and conceives once again. This fair occurrence is called superfecundation.
For superfecundation to happen, two things must happen. First, you have to ovulate twice. This is actually more common than you would think — this typically happens in about 1% (or more) of all ovulations. And second, you would have to have sex with multiple men within a few days (up to a week!) prior to ovulation. Sperm can last in the female reproductive tract for a maximum of 5 days, so the sexual intercourse will have to have happened a few days before ovulation to get the sperm time to fertilize the eggs.