Hi there, it's been more than a year since my last visit here. I noticed this thread on the homepage and the theory has stimulated my curiosity. So I read and reason along on it, but some points don't convince me at all.
I think so too. It would be a little too much for Sora continuing living with his Heart only and without Body nor Soul. But I suppose that neither the only Soul could be enough for it. I guess that Sora and Roxas are sharing a part of the same Body and Soul. Sora reborn as a human just at the same time as Roxas was birthing as a Nobody. Due to become a human again Sora takes away a part of his Body and Soul from Roxas who was using them for his own birth. The two boys became respectively a Human and a Nobody at the same instant and through the same Body and Soul.
Let's also think about what happens at the end of KH2, when Sora and Roxas return the same being: Sora would suppose to have two Souls at that time? I difficulty think so at the moment.
I'm pretty sure that the influence of Ven with Sora and the Roxas look is connected with something else than the Soul. Maybe the Ven essence exists in anyway within the Sora's Heart? Maybe something more has happened after the scene of a BbS trailer in which Terra (if I well remember) was looking at the young Sora and Riku playing on the beach.
The Heartless lose their Identity and their "Mind", more or less.... that's why they make their actions because of their own Instinct.
The Nobodies instead keep Memories and they are able to Plan and Reason their goals.
Anyway, I really don't know if could exist a 3rd being...
It might be. Each Organization member's name is an anagram of their original self's name with an "X" added.
Xemnas's an Ansem anagram just cause the original self of Xemnas called himself as "Ansem". However it's possible in the next game will be original selves of Organization member of course.
Just as user break said, you go to the Pride Land. You just lower Sora's HP in critical state. Equip Sora with Gull Wing Keyblade so go to the King's Den and go up to the Peak. Here you'll meet thousand Heartless (Rapid Thrusters) and ya'll get many EXPs defeating them by the Ability Experience Boost of the Gull Wing Keyblade.
I advice to you two more things: disable Donald Spells (expecially Cure's Spell) and able Sora's Ability "Experience Boost" if Sora has it.