Board Thread:Questions and Answers/@comment-2037710-20131106061354/@comment-6697950-20141012155240

Actually, what made him unique was that he could USE SWORD SKILLS while dual wielding. Anyone can dual wield anything they want to, if they so desire. But the catch is that you need a weapon skill for such a combination to use Sword Skills. Without them, a fighting style is pretty much rendered useless due to low damage.

Additionally, when Lisbeth saw Kirito putting on two weapons, she said:

"I couldn't comprehend his intentions. After having manipulated the equipment figure earlier, with the current system, his equipped status should be only the sword in his left hand; holding onto another weapon with his right hand should not be of any use whatsoever. On the contrary, with something that's considered an irregular equipped status like that, it would not be possible to activate sword skills."

Which implies that there were no other skills like that.