He means: 'Can you use Riku in Versus Mode?'
The answer is no, only sora can be used because the way Riku's deck works, someone would get an unfair advantage (cause Riku's deck depends on what world he's in, and you can chose the worlds). Also if it was Riku vs. Riku the battle would be pointless cause the Decks would be the same (but It could be a match of who handles the Deck best).
