Two Headed Snakes

It is difficult to draw the line between what is considered "one animal with two heads" or "two animals that share a body". With humans, dicephalic conjoined twins such as Abigail and Brittany Hensel are considered "twins", i.e., two individuals. This makes sense as there is a range of conjoinedness, and non-dicephalic conjoined twins may be barely-conjoined and separable by surgery, as was the case with Chang and Eng Bunker. Although the Hensel twins only have one pair of arms and legs total, each twin controls one side of the body's limbs. On the other hand, Syafitri, born 2006 in Indonesia, were given one name by their parents because they only had one heart. With other animals, polycephaly is usually described as "one animal with two heads". One of the heads, especially in three-headed animals, may be poorly developed and malformed, and not "participate" much.
Earliest known occurrence
The February 22, 2007 issue of the journal Biology Letters detailed the discovery of a 120 million-year-old fossil of a 2-headed Hyphalosaurus lingyuanensis (a type of lizard), marking the earliest known occurrence of axial bifurcation.