The penis is a male reproductive organ. Snakes have two penises, each of which is called a hemipenis. Together they are called hemipenes. Each hemipenis is associated with a single testis. The reason snakes have two penises instead of just one has to do with breeding and being able to mate with as many females as possible without diminishing the sperm count each time the snake mates.
Helpful Links:
Version 6.0 - VML Multiverse View