Dual Blades

Dual Blades (二刀流) is a Unique Skill in Sword Art Online. According to Heathcliff, who is in fact Kayaba Akihiko, this skill is awarded to the player with the fastest reaction time and the chosen player is supposed to be the one who will stand against the final boss (Heathcliff) on the 100th floor in the Ruby Palace, regardless of the outcome of the said battle. It was awarded to Kirito for being judged to have had the fastest reaction speed in the game. In addition, it also has its own set of Sword Skills unique to it.

In Sword Art Online, Kirito wields Dark Repulser and Elucidator for this sword equip skill. After his Dark Repulser broke, he tried to use Asuna's Lambent Light in place of his broken sword. In ALfheim Online, Kirito instead dual wielded his Black Iron Great Sword and Leafa's katana.

Techniques
Kirito estimates that there are slightly more than 20 techniques to the Dual Blades skill.

In the series, Kirito has been seen using the following:
 * Double Circular: 2-hit combo
 * Starburst Stream: 16-hit combo
 * The Eclipse: 27-hit combo

Timing Conditions

 * Attack Speed Bonus: 1.80x
 * Weapon Defense Bonus: 1.50x
 * Cooldown Time Bonus: 1.20x

Aincrad Arc
In Sword Art Online, the first time he used Dual Blades in public was against The Gleam Eyes while trying to save several members of the Aincrad Liberation Force from the boss, though he hesitated to use his unique skill until the boss created more havoc against the party members. Kirito explained to Klein and Asuna that although Kirito had discovered the skill a year earlier, he managed to keep his skill a secret from most players, except from Lisbeth, who he showed the skill to after he received his Dark Repulser from her, and probably Heathcliff as well, as the latter was the one who designed the skill. Because Kirito didn't know the conditions of unlocking the skill, he chose to keep his skill a secret to avoid unnecessary attention and problems, training the skill only when nobody was around to see it and even after nearly mastering it. However, the next day, his fears came true, as not only was his Dual Blades published on newspaper but many players chased Kirito out of his home while trying to ask him about the skill, forcing him to take refuge in Agil's shop, and Lisbeth teased him by saying that it was Kirito's own fault for publicly revealing the skill.

After the 74th boss fight, he later used his skill again while in a duel against Heathcliff, though Kirito lost the duel due to Heathcliff using the system assistance past its normal limits to help him win the duel. Kirito also used his swords against the Four Eyed Frogs and The Fatal Scythe in the Hidden Dungeon. He used it again in a second raid against the 75th floor boss, The Skull Reaper.

The last time he used it was against the revealed creator of «Sword Art Online», Kayaba Akihiko (whose in-game avatar was actually Heathcliff). Though Kirito knew that Kayaba was the maker of the skill and he had to use his own abilities rather than the game's skills, Kirito accidentally unleashed the 27-hit Dual Blades Sword Skill «The Eclipse» which proved to be a fatal mistake, as Kayaba effortlessly blocked every hit of the skill and his Dark Repulser broke on the final hit, leaving Kirito wide open to Kayaba's counterattack. This effectively almost cost Asuna's life, who sacrificed herself by shielding Kirito with her body. Kirito then equipped Asuna's Lambent Light, but poorly swung his Elucidator, allowing Kayaba to disarm his own sword, leaving him with the only one rapier.

Fairy Dance Arc
While Dual Blades skill is not available in ALO, players are able to wield two weapons, but unlike in SAO, dual wielding in ALO does not have a system-generated skill assist, nor is it influenced by the Dual Blades Skill stats. Since ALO's skill mechanics is based on the actual person's skill in using his/her weapons, the player has to actually learn and be proficient in using the said skill.

Leafa stated that there were some players who had tried dual wielding in ALO, but it needed a lot of practice and experience to actually be effective in combat, the same as in the real world, as there were only a few who could dual wield kendo swords very effectively. Kirito was able to duel wield in ALO efficiently mainly because of his experience of using Dual Blades in SAO.

When Sword Skills are introduced to ALO, the Dual Blades skill (and, therefore, all Sword Skills that were available with the Dual Blades skill) is among the skills which are removed by the new managers; when Kirito joins ALO and discovers that his SAO skills were also transported into ALO, he discovers that his Dual Blades skill has illegible text and is therefore corrupted. However, Kirito can use one-handed Sword Skills with each of his two swords. Using the Outside System Skill, Skill Connect, Kirito can activate a Sword Skill with one sword and, when that skill is finished and the cooldown starts, activate another with his other sword in his other hand. Doing this let's him avoid the cooldown time after each skill. Though not actually Dual Blades it was remarked by Asuna and Klein that this was extremely similar.

Trivia

 * Although the Dual Blades Sword Skills are not available in ALO, with the introduction of Original Sword Skills, Kirito has been trying to reproduce his old Dual Blade skills as Original Sword Skills. So far he has reproduced 3 of his Sword Skills, but he is going through hell to reproduce Starburst Stream.