In cats having two tongues is probably cause by a mild case of being 'diprosopus'--that is, having two faces diverging at the mid-line. On some cases only some of the internal structures are duplicated and the cat appears externally normal.
Initially the cause of this condition was thought to be incomplete separation of twins, but now it is thought to relate to the malfunctioning of a gene that controls facial development.
Pha Pathan (2004)
A black Persian kitten born in Malaysia was reported to have two tongues. Because of their size and positioning the kitten could not nurse and had to be fed with a syringe. No information is available as to whether the kitten survived and there are no pictures.
Max, a black and white cat is described as having two tongues. (Unconfirmed).
Double Tongue Cat (2009)
Some people create the illusion of having two tongues by surgically bisecting their tongues. Each half of the tongue is capable of independent movement. For example:
A recently discovered species of slow loris, Nycticebus kayan, has two tongues.