It didn't exlain it really, it was more of an educated guess. And it's possible that she did do it, and not even know she did or how she did it because at that time she didn't even know of her powers. Also, Sora and Roxas merged twice. In the begining to make Sora whole, and then at the end they had a complete merging. In a way, it's kinda like Ven and Vanitas in BBS. The battle between Sora and Roxas in TWTNW is similar to the final battle of Ven and Vanitas, except they were just fighting for control, not to destroy one-another.
It's funny that it's an "educated guess" since you have no evidence to support it. An educated guess would be thinking about the FACTS, which you haven't used yet, and getting something based off of that. And i think that Roxas and Sora fighting in TWTNW was more of a way for Roxas to find out if Sora was strong enough to take on Xemnas, and to learn why Xemnas picked him to destroy the Heartless and collect hearts in the first place.