The first known case of this condition was reported in 1609. Typically only one of the organs is fully formed, but there have been occurrences in which both penises are fully functional! This strange disorder affects an estimated 1 in 5.5 million men.
It's very uncommon for both penises in diphallus to be fully functional. Usually one of them only has rudimentary functionality.
While it might sound awesome, most people would rather not have it. In 2006, for instance, an Indian man in Uttar Pradesh had his second penis surgically removed
For more info about diphallus check out this scientific journal article. There's also a Wikipedia article on the subject. For more information on the recent example in India here's the full Reuters story.