According to the Wookieepedia article, the double-blade lightsaber aka saberstaff, Lightsaber Lance, Doublesaber, Lightstaff, or the Sith lightsaber. Was mostly a simple design of 2 joined lightsabers connected together at the pommel.
The doublesaber design was simplified in later years, retaining a single mechanism, but featuring an enlarged hilt. Whether this was an intentional design change or simply the result of contemporary manufacturers being incapable of replicating the original mechanism is unknown. However, most saberstaff designs were extremely simplistic, consisting of two separate lightsabers with connected pommels, rather than a single mechanism. This design, while less elaborate, also resulted in a considerably lengthened handle, though it otherwise remained the same.
As a reference, Asajj Ventress employed the 2 seperate lightsabers joined together to form the staff, given to her by her master Count Dooku in the Clone Wars series.
As to the specifics of Darth Maul's lightsaber staff I could not find, however from the article above the former does not specify that "single mechanism" is using just one crystal, it could be implied that the mechanics are singular like a normal single lightsaber, but uses two crystal's to ignite both blades. Not entirely impossible scenario when Darth Maul's lightsaber staff was cut in half, that one half would still be viable. As to the latter type of lightsaber staff design, it's basically two lightsaber hilts, so theres no reason why if the join was broken that one half/saber would still function.
