The presence of two fetuses in a uterus, developing from ova fertilized at different times.
This is known to be possible for some animals, such as the Eurpoean brown hare.
In humans, it is extremely rare, but there seem to be a handful of cases. Both children are born at the same time, so the conceptions can't be more than a few weeks apart if the second child is to have time to develop.
Etymology: Latin, fetare, to conceive.
Although the definitions I saw seemed to always refer to two fetuses, I assume the same word would be relevant if a fetus from a third fertilized ovum were introduced. Should such a thing ever occur.
I've been watching The Detour, and am wondering if this word will make an appearance in an episode soon.