It is one of the 10 unique skills.
Dual Blades is one of the two known unique Sword Art Online skills. Holy Sword, the other unique skill, belongs to Heathcliff (Kayaba Akihiko). As Heathcliff mentions, Dual Blades is a skill earned by the player with the quickest reflexes. The skill is supposed to be used in the final battle against the 100th floor boss. Heathcliff also reveals that Sword Art Online was designed to have 10 unique skills, however, there’s no reference to the remaining eight. The Dual Blade skill allows the user to fight using two swords and also grants access to special techniques. Kirito believes there are about 20 techniques associated with the Dual Blades skill. He discovered and used four of them. Dual Blades gives the users three bonuses pertaining to attack speed, weapon defense, and cooldown.
It is automatically unlocked by meeting a certain condition.
Kirito is seen using the Dual Blades skill for the first time during the battle against The Gleam Eyes. After the battle, he explains how he came in possession of the skill. About six months before, Kirito noticed an extra skill in the skill window. He didn’t know what unique skills are and he was clueless about why and how he got it. The Dark Repulser sword was forged to be used as a secondary weapon for this skill. Kirito decided to keep the skill a secret because he was already branded as Beater (combination of beta tester and cheater) and a new powerful skill would have raised even more suspicions.
Kirito revealed it so he could save others.
Kirito and Asuna discovered The Gleam Eyes 74th floor’s boss but they quickly retreated as they figured they have no chance of killing it. They were having lunch outside the boss room when they were interrupted by Klein and his guildies. Members of another guild, Aincrad Liberation Force, show up and their leader demands the map that leads to the boss’s location. Kirito gives it to him and they part ways. Sometimes later, Kirito, Asuna, and Klein hear commotion from the boss room. They head inside and see the Aincrad Liberation Force overpowered by the boss. The leader dies under their eyes. Kirito jumps into battle but realizes that he doesn’t stand a chance without the Dual Blades skill. He asks Asuna and Klein to keep the boss busy for 10 seconds so he can activate the skill and equip the Dark Repulser. Kirito defeats the boss with the Starburst Stream 16-strikes technique.
Dual Blades is used in combination with Elucidator and Dark Repulser weapons.
Elucidator is a one-handed longsword used by Kirito throughout Sword Art Online. It’s a black sword that Kirito carries sheathed on his back. Kirito shows Lisbeth the sword as a model for a new custom-made one. He mentions he got it as a drop from the boss on the 50th floor. As no other weapons in Lisbeth’s shop are as powerful as the Elucidator, the two go on an adventure to obtain the materials needed for a new sword. Lisbeth forges Kirito’s second sword, the Dark Repulser, from a Crystallite Ingot. The Dark Repulser is destroyed in the battle against Heathcliff.
Kirito created a custom Dual Blades version for ALfheim Online.
When the Sword Art Online skills were transferred to ALfheim Online, 10 were left out. Dual Blades is a skill that cannot be imported so it’s safe to assume that the 10 skills that didn’t make it are the unique ones. In season 2, during the Calibur Arc, Kirito reveals an outside system skill called Skill Connect. He uses the skill with two swords and even if it looks like Dual Blades, it’s not. Kirito is dual wielding and alternating between one-handed sword skills and that’s why it appears to be Dual Blades.