The original double-bladed lightsabers devised by the original Dark Lords of the Sith Empire were a sophisticated design, consisting of a single lightsaber mechanism with two sets of emitters, one on either end of the hilt.
The techniques employable with a double-bladed lightsaber were extremely limited, as the second blade rendered many positions and movements impossible to execute, or required the wielder to twist oneself awkwardly around the weapon to do so. Many techniques required the user to perform elaborate flourishes in order to move the staff around their body, leaving them open, and any sort of fighting method that focused on power or penetration was difficult to apply.
Due to these many drawbacks, most duelists opted for the standard, single-bladed lightsaber as opposed to this specialized weapon. However, this provided most double-bladed lightsaber wielders with an additional edge, due to their opponent's lack of experience with the weapon, though even this edge could be countered by something as simple as quarterstaff training, preparing a duelist to engage someone wielding a saberstaff.