~Vanguard Nigeria. Sunday, January 15, 2017.
It is often assumed that for twins, both eggs were fertilized during a single act of intercourse. However it is quite possible for one egg to be fertilized during one act of intercourse, and the other during another if the woman has intercourse with two men within hours.
Normally, women ovulate only one healthy egg per cycle. Fraternal twins are born when women ovulate two healthy eggs and both get fertilised. But it is also possible for two eggs to be ovulated during the same cycle and fertilised at different points within the five-day fertility window resulting in twins. This process is known as "superfecundation". What this means is that each egg can be fertilised by different sperms.
If a woman ovulates two or more eggs and she has sex with more than one man while she's fertile, "heteropaternal superfecundation" can occur, if the eggs get fertilised by sperm from the two different fathers within the same ovulation period.
Medical research shows that when a woman has sex with two men within the same ovulation window, it can result in bi-paternal twins, that come when a woman releases two eggs during ovulation instead of one - and both eggs are then fertilized.