I finally figured it out.

[Captain Arrowguns 466]

One of the debates about what will happen in KH3 is whether Roxas and Ventus will be the same person when they come back, or be two different people entirely. Believers in the latter state that they both have different personalities. I finally came up with how these two characters can have different personalities, and still be the same person. Before the events of BBS, Xehanort extracted all of Ventus's darkness in order to create Venetus, thus giving Ventus a heart of pure light. But when Venitus was destroyed, his darkness returned to Ven's heart, thus he had both darkness and light in his heart whis he's Roxas. In other words, Ven has a heart of pure light (for a while, that is) while Roxas doesn't.

[Javelin434 3,164]

But... Vanitas looks like Sora?

[RoxSox 3,593]

I believe that Vens heart laid dormant in Roxas body. Influencing his appearance, but not who he was. I think Roxas grew his own heart, like the others. Thus making Ventus and Roxas seperate people, seperate hearts.

 

The complication is the fact that Roxas doesn't have his own body... but evidently, hearts can develop human form.

 

Want proof of that? Think back... Sora was a human form heart from Soras sacrifice until KH2. Roxas was his body, but Sora appeared as a person... meaning... put it together.

[Indecypher 495]

I'm not sure I understand, is this the "Roxas is Ven without memories" theory?

[AxelRoxasXionKH 105]

I think that they wheren't the same to begin with, and they've never been.so them excisting next to eachother shouldn't be a problem.Ventus only restored a little part of sora's heart. Roxas has that part too right? if Roxas couldn't excist next to Ventus because he has a part of his heart, that would mean Sora couldn't either.

[HarLea Quinn 26,501]

Roxas is his very own person to begin with. He may of had Ven's heart inside him but it was dormant or sleeping pretty much .He didn't even summon Ven's keyblade till Xion's death awakened it through emotional turmoil .Then , since his body lacked it's own original heart , over time Roxas grew his very own based on his very own memories..In DDD we see this verified and Ansem the Wises' hidden data hints we can use it to bring back all Sora's fallen comrades.