Also worthy of noting, dual-wielding isn't strictly limited to a requirement of physically harboring multiple hearts. As special of a case as he might be, Roxas is the prime example, being able to wield Sora's keyblade without harboring his actual heart (the general consensus is that Sora's heart resonated within him). As far as we know, Roxas only had one heart, that being Ven's. That tells us that other characters can dual-wield without physically harboring multiple hearts, albeit under...complicated circumstances.
You state it yourself. Roxas is a
special case who doesn't work like the others. It is plainly stated that Roxas "existed closely with Sora" from the start as well as the sheer fact that with Roxas and Sora the Nobody and original persona coexisted with each other, which no one else, not even Xehanort, had in play.
In Chain of Memories Vexen could draft a card from
Roxas' memories of Twilight Town from within Sora's heart, showing just how close those two actually existed. It's that very close bond which eventually results in Roxas unwittingly absorbing Sora's unhinged memories after Naminé messed them up and what Xemnas exploited to have Xion absorb them from there.
Furthermore, Roxas never
dual-wields until late in his "life" when he activated Ven's keyblade after absorbing Xion. By that time, his new heart would already be pretty far along and count as another heart present, even if the special circumstance of his direct connection to Sora's heart would
not be a factor. Xion's grown heart could also count as a factor, so everytime Roxas does indeed dual-wield, he has two hearts present. Ven's and either his new one or Xion's, with Sora's heart all the while supplying the ability to wield in the first place.
Early on, Roxas wielded only one Keyblade (Sora's), and the reason he could summon it was because of his connection to Sora's heart while the reason he could
continue to hold it was because he carried Ven's heart.
I don't remember which Ultimania it was exactly, but it was pointed out in one of them that "you need a heart to wield".
All this means the case can only be applied to Roxas alone and not other characters, because no other character had his/her Nobody and the original persona coexisting at the same time.
It
may be a possibility that the same logic could be applied to Kairi and Naminé due to the similar circumstances of them coexisting, but in their cases there are no Keyblades to wield.
You know what would be funny? That during all this time Xehanort has been seeding people after taking over Terra's body, Terra has also seeded the same people Xehanort has seeded. So Anyone after Braig has, not one, but two seeds inside of them. I can see Terra playing this card, but it would only be a temporary measure, seeing as how Xehanort will battle the Seven Guardians of Light alongside his other twelve Seekers of Darkness, and he will finally wield the chi-blade.
One can argue that Roxas could wield a Keyblade because he had Ventus' heart inside of him, but could wield Sora's because Sora's heart is VERY linked with Ven's. One could also argue he could dual-wield because, after Xion merged with him, perhaps he began cementing his own heart? Nobodies can grow hearts with the pass of time. Well, Nobodies and anything. So perhaps both Xion and Roxas were beginning to form a heart, but after Xion merged with him, he finished his own heart...?
I always thought that when Nomura said Xemnas could wield a Keyblade, but decided to just not use it, he may have been hinting at Terra, since he said that Xemnas choosing to not wield a Keyblade was going to be something he was going to leave loose until KINGDOM HEARTS III (assuming he even touches Xemnas back besides being a Seekers, seeing as how Xemnas has been reduced to just being another Xehanort).
Sorry to say that, lol, but for a scheme that savvy and sophisticated I don't think Terra is quite clever enough, not to mention that just extracting and moving a heart is a technique only true Keyblade Masters know/get to learn. To actually learn to
cut your own heart into pieces and sent them elsewhere I'd imagine it needs more knowledge than the average things normal Keyblade Wielders know.
The clash between the seven and thirteen maybe preordained, but the result coming of that clash
isn't. It isn't even sure if the clash will really produce the actual, flawless X-blade this time at all nor is it set in stone that if something comes about it, Xehanort will get to wield it.
It's both actually. Roxas had the ability to wield in the first place because he was born from Sora's heart and had that close link with Sora from the start. Ven's heart being present allowed him to actually summon and swing Sora's Keyblade around like nothing because a heart needs to be present in order to properly wield the Keyblade.
That's the thing, Roxas only wields two blades in the later stages of his physical appearance in the series, at a time when he most likely has already grown his own heart. After absorbing Xion, who most likely did the same, any combination of the three hearts involved (Ven/Roxas, Roxas/Xion, Ven/Xion) would meet the condition for dual-wielding either way.
The only thing Nomura said about the question if Xemnas could wield was that he would like that "to remain a mystery", he never said anything about KH III in regards to this topic or hinted otherwise at planning to resolve the mystery.
Just letting it "remain a mystery" can also mean "this isn't going to be addressed at all!".
Although it's pretty much certain that Roxas had fully developed his own heart by the end of Days, as far as I'm aware it has never been "chosen" to be able to wield the Keyblade since he doesn't have a Keyblade of his own.
That he doesn't have a Keyblade of his own doesn't matter a thing.
Riku also didn't have one after the Kingdom Key chose Sora for good and yet still he could wield i.e. Roxas' Oblivion because his heart has the
ability to wield.
Roxas' newly grown heart would also easily have the
ability to wield since it originates from the heart of a qualified Keyblade Wielder: Sora.
Vanitas' new heart could also immediately wield because it was derived from the heart of another wielder.
