You have multiple cases of two-headed malformed bodies that happened in the real world (from lizard to sheep). There are even conjoined human twins living in USA sharing a single body with two heads. I'd advise you to simply ask Google for examples.
Edit (thanks to Pelinore): Every instance of this on record is down to developmental problems in the womb, most often identical twins that only partially separated or parasitic twins that weren't fully absorbed by the "surviving" twin. None have been due to gene mutations.
If you are looking for a creature with a brain that is spread across the body with several main parts, octopuses hold a part of their brain in each of their tentacles, if memory serves.
Your question is a bit unclear about the "consciousness" part, since it's a human trait.
Edit to clarify: The first paragraph is about a body with two heads and two brains, which produce two separates "beings" sharing the same body.
The second is about a single being but with multiples brains (9 in this case).