Conjoined twins

Two years ago, the BBC reported the discovery of the first dicephalus (two-headed) lizard in the fossil record (pictured). The anomaly is not infrequent among modern reptiles. It is less common among mammals (see these videos of a two-faced kitten and a two-faced adult cat). Dicephalus humans are extremely rare and only four sets have lived to adulthood. The modern counterparts to Giovanni and Giacomo Tocci are Abigail and Brittany Hensel, who have now reached the age of 16. While the Toccis were never able to coordinate enough to walk, the Hensels have walked, tied their shoes, ridden a bike, and swum from an early age--and now have their driver's licenses!