Roxas came from Sora in the first place. So they'd return to being one person.

[He frowns. He knows now that since Roxas had left the Organization that he'd eventually run into Sora. But he didn't know that at the time, and even now, leaving it up to chance feels like it might have been risky.]

So I went to get Roxas, and we fought. Which I guess is to be expected. He beat me, by the way. He's tough. But I had the darkness powers I'd already developed, and I took them and let all the darkness out at once, and I overpowered him.