User blog comment:Akumation/Episode 4 - Kirito's not the first to dual wield?/@comment-24281887-20151106040848

It's most likely the way the Axe's appearance was programmed. Some MMOs do this, where one item equipped appears on your avatar dual-wielded. SAO probably wasn't an exception, but given how the axes are the same physically, it's safe to assume that the pair of axes is just one weapon coded to appear in both hands. Besides, dual-wielding in SAO involves two seperate longswords.

Also, SAO Abridged. Hilarious AF.