As the name "two swords" suggested, the fighting style was about fighting with two swords, one in each hand. What made this style special, was the simultaneous usage of these swords as offensive and defensive tools without allowing these aspects to cannibalize each other, enabling the swords into working as weapons and shields at the same time.
As the name suggested, would-be users needed to learn how to fight with two weapons simultaneously at all.
The entire style required a natural talent in the form of perfect balance between the users' two hands. Those who lacked this talent, first learned the basics and then tried to master it either with more diligent training or retorting to magical help. Regardless, whether one did or didn't have the talent, due to its difficulty, to master this style, trainees needed to live in a station, which allowed them the luxury of formal education in the art of combat for example in the form of private lessons from a weapon master.
The style was difficult to learn and only those with both the talent and the luxury of formal training were capable of truly mastering it. Drizzt Do'Urden had both and was probably the most famous user of this style.
The style was based on the Two-Weapon Fighting and Twin Sword Style feats in 3.0-edition Dungeons & Dragons.