Pretty sure it was because of Ventus and Roxas' own heart. It couldn't have been Sora as Sora's heart was walking around in Castle Oblivion.
What I meant was that Roxas in a way is Sora's body and Ventus' hearts in one being. Remember that even Riku explained that he could wield the keyblade because he was Sora's nobody, and he could double-wield because Ventus was also a keyblade wielder. This was explained on the BBS Ultimania long ago.
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