"Star Wars" has its own history that extends beyond the films to video games, books and comics. Thousands of years before the period in which the films are set, the Sith Lord Exar Kun constructed one of the first lightsabers with two blades.
The double-bladed lightsaber is a good match for Sith aggressive nature. Unlike the single-bladed variant, the double blade creates a zone of intimidation that can project far enough to offer some rear protection. One of the blades can also be turned off for greater versatility.
From watching the Darth Maul versus Obi-Wan Kenobi fight scene in "The Phantom Menace," the sheer bulk of the Sith weapon is a drawback, and from a cold start, the weapon cannot be swung quite as quickly as a single-bladed lightsaber.