Roxas and Xion have their own budding hearts so they should be able to wield their own Keyblades, after they separate from Sora then it's just a matter of being chosen. Dual wielding is too signature for Roxas to lose so I think Nomura will whip up some heart mumbo jumbo to say that he can still dual wield because of his strong connection to "X" character or some shiz. Roxas will probably still have Oath Keeper and Oblivion, Xion's Keyblade was a fake and I have no idea what they'd give her. Possibly something entirely new.
On the subject of dual-wielding I'll still stick with my theory about Roxas just having one Keyblade, but that one being his personal one which can be split into two blades for pseudo-Dual wield like Cloud's Fusion swords from FF VII Advent Children. Would be better than some ominuous heart-mumbo-jumbo also in the aspect of dual-wielding hopefully being unique for Roxas. Sora should lose dual-wielding and Ventus should certainly not gain it, as "heart-mumbo-jumbo" might actually indicate that they can all three do it due to triplet-thingy-whatever between their hearts.
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Xion's keyblade wasn't even Sora's keyblade per se as Roxas could use, since Riku was surprised that Roxas used a real Keyblade compared to Xion's fake one, which Riku even called "a sham" in-game.
Xion's "keyblade" was just a fake one with nearly the same abilities, but still different enough for Riku (in-game) and Nomura (in-interview) to point it out.
So Xion will need a true Keyblade to choose her anyways (Roxas needs that too, but Xion's case is more obvious).
Since the main requirement for getting a Keyblade is a strong heart, I have little doubt that they will be both qualified enough to wield.
The main question would be then however when a personal Keyblade chooses them.
Maybe it will be Lea's turn to taunt them about it then, telling Roxas and Xion that they apparently do "something wrong with the flick of the wrist".
