Hard to keep track of 'em all.
Ventress, a misguided force adept according to Kenobi at some later date. She in known by most for having dual wielded, though she started out with a single blade. Taken from her slain master, while in grieving.
Which has its connection: She received her dual blades from Count Dooku. The blades belonged to his former apprentice, Komari Vosa some time. Fallen to the dark side, Vosa became a target and a prey for bounty hunter competing for both a large sum of money and immortality through becoming the specimen for the clone troopers. So you can add her to dual wielding.
Speaking of Kenobi, he had another dealing with yet another who had fallen to the dark side: A'Sharad Hett born of a Tusken Raider tribe, he was the son of Jedi Sharad Hett. A'Sharad Hett was a dual wielder as well.
@adam: Thanks. Hey, regardless of intent you still helped me remember something.
Edit: Ah yes, and another one is Jaric Kaedan. Seen in the lost suns comic, wielding dual blades while near Satele Shan, preapring for a fight. Noted as a "living weapon" and a master of Juyo-Kos, apparently another name for the seventh lightsaber form. Perhaps the additional "kos" name refers to the dual wield variant?
Juyo-Kos, J-K; J'K, Jar'Kai.