O.O
This topic is interesting.
To me it goes like this.
Sora is a regular keyblade having moron on an adventure.
He becomes heartless
Then we have the emo Roxas who knows not who he is.
I believe even though theres a more precise term but it comes down tot his basically.
You take Sora divide him by two
Add the fact he can wield and add the fact Roxas can wield
Add the fact Roxas has some odd dreams and memories of Sora.
And what do you get?
The ability to wield two at once.
Its hard to explain but basically.
He is Sora and Roxas giving him the ability to use two keyblades
But the keyblades of of Soras memories