It's explained already. The time traveling Younganort has part of Oldanort's heart in him already.
That's why he can use the Keyblade too.
Is that the case? Don't remember that being shown in KH3D. Thought Younganort only had contact with Ansem SOD.
And even if he did, I remember in one of the secret endings, Braig was talking to Younganort that "even he doesn't know what the old man is thinking", but if he was possessed by him, surely he would?