I don't think it's clear as of yet what, precisely, Young Xehanort is doing in the Realm of Sleep. His appearance in the trailer was somewhat anomalous.
However, we can't be sure precisely what will occur given that Xemnas and Ansem SoD have both been defeated, because we're dealing with two Hearts. Presumably, Terranort/Apprentice Xehanort would be the Somebody that comes back, but it's hard to say because the interaction of two separate, active Hearts in the process presents an unusual situation. Unlike with Sora, where Ven's heart is docile and therefore most likely returned to him along with Roxas, Terra's Heart is (to our knowledge) fully conscious. Furthermore, the point where the heart and body rejoin may have served as some kind of opportunity for Xehanort to enact another phase in his plan. If, for any reason, Terra and Xehanort's hearts did not both merge back together (presuming they were ever separated; it still isn't clear how their Hearts were divided up in the process of creating a Heartless and a Nobody), we may have Young Xehanort to represent Xehanort's Heart and another entity to represent Terra's.
Given that this is happening in the Realm of Sleep, though, it's all more or less up in the air.