In Master Form, he is holding one that hovers in his hands, don't forget. I'd say the only form where it can be clearly seen, defined and judged as Sora being ambidextrous is Valour Form.
That's only when he isn't using it. When he is actually attacking someone he holds the Keyblade. Though it doesn't really matter how many times it is shown he still has Wielded a Keybladed in both hands which is the point. Being captious is a waste of time because, it's not proving anything other than my point.