Nifty question and the answer is yes, it could happen, going by the basic rules of mutation. Going entirely by bio-science however, the probability of such a creature being viable is somewhat less likely than Curiosity topping a rise in it's Martian meanderings and taking shots of a group of 12 foot protozoans engaged in a baseball game.
Not on THIS planet, we won't. You have to understand that such an animal-a vertebrate-with a head at each end would lack a rectum or cloaca, and probably a digestive system as well. Like The Snark said, we'd be more likely to discover, via the Curiosity Rover, a football game being played on Mars by 12-foot tall protozoans. People who actually believe that a snake can have a head at each end also believe that snakes do not have to eliminate waste, or that they do so through their mouths, along with giving birth/laying eggs through their mouths. If you live down South, you will hear some of the most outrageously stupid things about snakes, and people actually do believe those things! The level of ignorance about this group of animals is nothing short of astounding.
Comparable to going over to Europe and people believing dragons are real. It is mythology, nonetheless. It is quite dumb that people never think to learn about our fellow vertebrates. No love for the snakes or lizards. One day, we'll live in a world of iguanas with stingers and "poisonous" snakes with the anatomy of a planarian. Oh, but we'll still have cute, painted turtles, and tie-dye frogs, right?