I didn't read all of the pages, but I'd just like to point out that everyone is stressed out about dual wielding when we NEVER got a proper explanation on how the Keyblades in general work in the first place.
Originally back in KH1 there was only one Keyblade: THE Keyblade, the one Sora used. We knew it was meant for Riku but then Sora earned his. Mickey got his, but at the time we didn't know he was actually a proper wielder too, just that his Keyblade was from the Realm of Darkness.
Then Days had Sora and Roxas SIMUTANEOUSLY using the Kingdom Key. Not Ventus Keyblade taking the form of the Kingdom Key, but the actual damn thing. And then Xion and Roxas after Sora went into hibernation, and then Xion gets her own Keyblade.
Birth By Sleep then happened and we found out about the inheritance, but it was never properly explained how it worked either: With Kairi's it seems like what she got was the power to wield one and not Aqua's Keyblade as a whole, while with Riku and Terra we know it's his actual Keyblade being passed on. And again, he uses it just fine when he comes back even though the actual thing is with Sora right now.
X and Union X show us that the MoM forges the first Keyblades as an imitation of the Chi Blade, but we never get proper explanations for that whole method either.
My point being: we all just kind of go along with the justification that people dual wielded in the past because they had more than one heart within them, but it was never flat out stated that this is the only way to do so or anything.
We have seen new Keyblade users pop up without having to go through Inheritance cerimony nor having their Keyblades forged by a Union Leader (Sora, Data-Sora, Xion, Repliku, Lea, theorically Roxas as well).
We barely have any rules explaining how the Keyblade work or what the powers of a Master are because Birth By Sleep and X never took the time to do so and the entire Xehanort saga had Sora do things without the guidance of a master.
This isn't even a situation where Nomura has to retcon anything because there were never any stablished rules in the first place, just a bunch of fan assumptions that we just kinda took as rules with time.
(And I'm not saying this is wrong because I definitely did that as well and it's the game's fault in the first place for not stablishing things)