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Is Sora dual wielding Oathkeeper and Oblivion in this?
Well actually, you can see that RIku is holding the Oblivion in the first part of the scene, when Xemnas is summoning all those orbs to shoot down at them.
But after that, when Sora and RIku shoot the beam of light to kill him, Riku just has hand on Sora's Oathkeeper, while Sora is holding the Oblivion in his other hand behind his back.
It was done in the end of KH2, only he placed Riku's Way To The Dawn over his Keyblade to fire that beam of light.