Manny and fanny, who are fraternal twins, were thus formed when

Fraternal twins typically happen when two fertilized eggs are rooted in the uterus wall at the same period or time. When two eggs are individualistically fertilized by two different sperm cells, fraternal twins is the end outcome. The two eggs form two zygotes, therefore the terms dizygotic and biovular. Fraternal twins are, fundamentally, two normal siblings who come about to be born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like normal siblings.