Sara Ketto. Killed by the newly christened Darth Vader. She dual wielded.
Exar Kun invented the double bladed saber in his sith days. He was a great duelist single and quite proficient dual wielder.
Kao Cen Darach was also a dual wielder but despite his supposedly superior skill and experience with the variant, he was still no match for the brute force of Lord Malgus.
Aryn Leneer also had some proficiency in dual wielding in Deceived, though initially she started out single wielding.
I personally think this style is underrated. Purists (IMO conformists) used to win the popularity vote with single bladed claiming dual wield is excessive, lacking elegance, too barbaric, and blah-blah-blah. Funny we didn't see this complaint about Maul with a double bladed saber. Whether people like it or not, it's a viable style and it's here to stay.
Even if on cutscenes in SWTOR only the primary blade is used, people like it enough to use it.
That's right, Bixby Snyder folks.