I think the problem here is the statement made in KH I over the fact that residents of a world should not know there are other worlds outside (was it Squall/Leon who said it, dunno...?) and the possible multiple meanings of "outside world".
World is a term used very broadly in the Kingdom Hearts series and most people I know associate the term "world" synonymous with "planet", but that doesn't apply here.
However, to draw a parallel, when Columbus arrived first in America the native population also thought of him and his cohorts coming from another world, where they in fact hail from the same "world".
So "outside world" could also mean anything beside the island group Riku and Sora live on...if the Destiny Islands world even has any landmasses beside that island group...
Perrine once said that, for poetry and scientific hypotheses alike, when there is no clear answer to an unknown, the conclusion with the least number of assumptions ought to be the truest one. This can apply to theories outside of the scientific area, and as such, may apply here.
Like you said, we have no indication on whether or not Destiny Islands has landmasses outside of the island Sora and co. live on, but we do have indication that there are other "worlds" in the universe (which is also referred to a much bigger "world").
It's fairly certain that Riku was referring to other worlds when he was talking to Terra.
Sephiroth0812 said:
If everything would be spilled out clearly the series would lose much of it's mystery and therefore over time become dull.
Which is exactly why powerre is SoL.
Sephiroth0812 said:
I remember Nomura saying somewhere that he chooses to leave certain things unexplained so that fans of the series can speculate and come to their own conclusions...even all the things written in Xehanort's Reports, the Ansem Reports (of Apprentice Xehanort) or the Secret Ansem Reports of AtW can't be taken as 100% correct facts since they're written by in-universe characters and AtW himself admits that most of his theories came out false.
Also I don't believe MX or Apprentice Xehanort to be omniscient...
I've to admit, without these little "inconsitencies" and mysteries the series would only half as interesting.
Well... right. Nomura intentionally keeps in inconsistencies and mysteries to keep us guessing, and therefore interested.
Also, while Master Xehanort is not omniscient, when he says that Eraqus's Heart is inside of Terra, it's to be accepted, as there's nothing to disprove it.
tbh with the duel wielding thing... can someone powerful enough not just cause two of their keyblade to apear ? i meen yeah the whole xion thing did represent herself in the second roxas keyblade but, since keyblades can take any form (so long as the user is strong enough ) the why can someone split their keyblade into two objects ?
Nomura said in an interview there is a Keyblade out there for every eligible Heart--that is, for every Heart that has the qualities to wield the Keyblade. To split your own Keyblade in two, you'd probably have to split your Heart in two, and at that point, unless your Heart(s) gained enough strength to each function as individual hearts, your Keyblade would probably be about half as strong as a normal Keyblade, or would disappear/break entirely.
I assume splitting a Keyblade is possible, but I also imagine doing such a thing would require a very, very resilient Heart.