sora is weird, because he's basically everybody. it might be easier to start off talking about axel...
lea was a somebody! you get to meet him in the game BBS, and sometime after that he and his friend isa (saix) became "nobodies."
when you become a nobody, your body retains your memories (a nobody) while your heart becomes a heartless.
there was some rumors going around in the early days that i'm pretty sure was never confirmed as canon, basically saying that in order to become a somebody again, both your heartless and your nobody had to be defeated (by a keyblade most likely?). a friend of mine named kupomog and i use to joke that axel's heartless must have been dark thorn:
that guy! haha. that is also not canon, it's just that the hair is a bit uncanny so the thought was funny.
but roxas and namine aren't really sora's and kairi's nobodies. the hypothesis gets a little convoluted here...
roxas and namine both look like a guy named ventus. you also meet him in BBS. he's a cute little mofo who will break your heart, just like roxas.
at the end of BBS, certain events lead to ventus and sora linking their hearts. this is hella useful as it preserves ventus' unconscious body out there somewhere, AND gives sora his dual wielding keyblade ability!
regardless of ventus' looks.... roxas and namine aren't really ventus' nobody either. they were both created when sora released his heart and kairi's heart at the end of kh1. kairi and sora both, however, returned to being somebodies shortly after this event.
so the two, roxas and namine, now exist entirely outside the same conditions axel did. neither of them have their "originals" memories, neither of them exist in their "originals" body, and neither of them can really return to being one with their original.
this is because they have coinciding existences. when axel and his heartless returned into being lea, lea came into existence again at that point.
but because sora and kairi were existing PRIOR to the final merger of nobody/heart that roxas and namine didn't really do either, their splintered personalities grew into being different people.
i still have doubts that namine had anything to do with kairi at all, but as for roxas - that's where the whole revelation in kh3d comes from. sora relents to roxas that roxas was his own person, melting the hearts of probably only me.
WHICH BRINGS UP A TON OF OTHER CONFUSING STUFF!!
like.. when sora became a heartless and roxas was created, where did sora's body actually go? we saw him floating in the darkness, but i'm pretty sure that was just his image. his body is just spurned into existence again when kairi reaches his heart!
what would have happened if we could have reached leas heart before defeating axel? would lea and axel have become splintered personalities as well?
i would have thought a definite no, citing ventus as the pure reason for roxas and namine, except for the missing body of sora.
mainly what you need to understand about kh:
none of it will ever make any sense. any theories you read on the internet are basically concentrated bullshit. nomura will continue to do whatever sounds best at the time and drive us all insane trying to reason it.