Superfetation happens when twins are conceived weeks apart from two separate ovulations and from two different fathers or the same father, the second one after the first has already implanted. Superfecundation are twins with different fathers but ovulations the same day.
Superfetation happens when twins are conceived weeks apart from ovulation happening on different days. For this to happen, two things have to happen:
1. You have to ovulate twice (dizyogtic) several weeks apart
2. You have get pregnant again after the first fetus has already implanted
Multiple ovulations happen in about 1% or more of all ovulations, more often so when you take ovulation-inducing medications, but they usually happen wit a day of each other.
The result of two regnancy happening weeks apart is that the two twins are in different developmental stages and one fetus is more advanced in pregnancy then the other. This is an extremely rare event in humans and has been reported only a few times. Superfetation can happen with the same father or different fathers.
Superfecundation on the other hand happens when twins are conceived by different fathers, usually within a couple of days from having sex with different men but from ovulation happening the same day.