There is some speculation that the inbreeding of snakes in captivity increases the chances of a two-headed birth. One or two beings? It is difficult to draw the line between what is considered "one animal with two heads" or "two animals that share a body". Abigail and Brittany Hensel, born in 1990, were given two distinct names at birth. They identify as two people, and are recognised as two people legally and socially. On the other hand, Syafitri, born 2006 in Indonesia, were given one name by their parents because they only had one heart. In early Germany, conjoined twins that could not be separated were legally considered one person. Millie and Christine McKoy were often referred to in the singular, including by themselves, with the name "Millie-Christine", as well as plural.In Peter Mogila’s 17-century Catechism, the following instructions are given for baptism of polycephalic infants: should there be distinct heads and distinct chests, this means there are separate people each of whom must be baptised normally; if the heads and chests are not completely distinct from each other, however, one person must be baptised normally but baptism of the other(s) should be preluded by the formula "if not already baptised".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.
Segmental odontomaxillary dysplasia is a painless, unilateral enlargement of the upper jaw. The cause is unknown, and the disease affects the jaws, teeth, and adjacent soft tissue. The premolars may be congenitally missing, and the primary teeth (baby teeth) may be smaller than usual. It is a relatively recent discovery. Description
It is a rare developmental disorder that affects maxilla, also known as the upper jaw in most vertebrates. The disorder is often diagnosed in early childhood. Since its original description as hemimaxillofacial dysplasia by doctors in 1987, less than 40 cases have been reported in the English literature. Epidiomology
All cases reported appear to represent sporadic occurrence. There is no specific inheritance pattern. The male-to-female ratio of affected is 1.8:1 and is often diagnosed before the age of 9. The disorder affects the right and left sides of the maxilla almost equally. References

Kahn, Michael A. Basic Oral and Maxillofacial Pathology. Volume 1. 2001.