My theory has always been that Xion, Roxas, Ven and even Namine can all exist separately if they get creative. First, return Ven's heart to his body. Then use a combination of the replica program and Ansem's memory/data pods to create artificial bodies for the memories of Roxas, Xion and Namine that are inside Sora and Kairi. Yes, it would be sort of weird, but it seems 3D is going to revolve around (in some way) the fact Sora has too many hearts/people/memories/data inside him. What better way to solve that issue then to remove those things and place them in another vessel.