Well in that case, Sora sharing Ventus's heart is quite similar to Master Xehanort taking control of Terras, 2 hearts, one body. And if Xehanort losing his heart to darkness and becoming Ansem SOD and Xemnas, not all disimilar to Sora, ok brought on by a keyblade made from hearts of light, still resulting in a heartless/nobody. Its all too complex. Id still say Roxas did kinda look like Sora. I dunno, maybe all things will become clear down the line, but the way the story is written so far i can't see why Xemnas can't wield a Keyblade if Roxas can. :S