Dual Blades

Dual Blades (二刀流) is a Unique Skill in Sword Art Online. The 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 on the 100th floor, 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. Kirito wields Dark Repulser and Elucidator for this sword equip skill.

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

Sword Art Online
In Sword Art Online, Kirito was able to use his Dual Blades skill very efficiently. The first time he used his unique skill in public was during a major boss raid while trying to save several members from the Aincrad Liberation Force. Although Kirito had discovered the skill a year before using it in public, 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, he chose not to use it against monsters, unless in case of an emergency, as he didn't like the attention the skill drew. 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 during a second raid against the seventy-fifth floor boss and, finally, he used the 27-hit Dual Blades Sword Skill «The Eclipse» against the revealed creator of «Sword Art Online», Kayaba Akihiko (whose in-game avatar was actually Heathcliff) however, using this skill proved to be a great mistake, as Kayaba was able to dodge every hit of the skill due to being the one who designed it, therefore, knowing each and every move of the Sword Skill, and the skill left Kirito wide open to Kayaba's counterattack.

ALfheim Online
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. 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.