As the name "two sword" suggested, the style was based around fighting with two weapons, one in each hand. What made this style special was the simultaneous use of each of these weapons for both offense and defense, that is, to attack and to parry, using each weapon like a shield.
As the name suggested, would-be users needed to learn how to fight with two weapons simultaneously. Ideally, they would be completely ambidextrous, but this talent was very rare. Those who lacked this natural talent first learned the basics and then tried to master it either with more diligent training (such as under a house weapon master) or resorting to magical aid.
The style was difficult to learn and only those with both the natural talent and the luxury of formal training were capable of truly mastering it. Drizzt Do'Urden had both and was probably the most famous master of this style. Apart from him, there were only a few talented drow in each drow city who practiced it.
This style is based on the Ambidexterity, Two-Weapon Fighting, and Twin Sword Style feats in 3.0-edition Dungeons & Dragons.