In Roman mythology, Janus was the god of gates, doors, doorways, beginnings, and endings. Now we have his namesakes, the month of January and the caretaker of doors and halls: Janitor.
Janus - used to symbolize change and transitions, the progression of future to past, of one condition to another, of one vision to another, the growing up of young people, and of one universe to another. He was also known as the figure representing time because he could see into the past with one face and into the future with the other – and he's the representative of the middle ground between barbarity and civilization, rural country and urban cities, and youth and adulthood.
Also see Diprosopus, a rare craniofacial duplication condition. It's congenital.