There are many Jedi who prefer to use two lightsabers at once in combat. Some use one for defense, and one for assault, while others use the additional weapon to press their opponent's defenses further. In any case, the use of two lightsabers at once is a very difficult task, require exhaustive and specialized training.
Any Lightsaber form of combat can make the use of a second lightsaber, although several forms accomodate it better than others. Shien and Sokan have special techniques that only can be accomplished with two sabers, and Jar'Kai Niman is a form built entirely around the efficient use of two weapons.
The use of Two Lightsabers at once is restricted to those who have reached the rank of Krath Pontifex, Sith Warlord, and Obelisk Primarch. Those below this rank are not permitted to train with them, nor display them on their dossier.