Superfetation

The formation of a fetus at the result of an impregnation occurring after another impregnation but before the birth of the offspring produced by it. This is possible only when there is a double uterus, or where menstruation persists up to the time of the second impregnation.

fertilization of a second ovum after a pregnancy has begun; results in two fetuses of different ages in the uterus at the same time; "superfetation is normal in some animal species"