In Kingdom Hearts 2 how do you fight Roxas

You don't actually fight him, but you see a short cutscene of Sora and Roxas fighting each other in The World That Never Was. Eventually, Sora wins and Roxas disappears.

But in Kingdom Hearts II Final Mix, you can fight him. I don't know how to get to him, but I saw some videos on Youtube of Sora fighting Roxas.