What is it with this inaccurate headcanon that the Oathkeeper and Oblivion represents anything with Roxas or his relationships? He got them because he's Sora's Nobody, they were the best Keyblades in KHI (also popular), and they make a good contrast with light and dark. That's it.
He could dual wield because of Sora and Ventus.
Now about it there being two Kingdom Keys, well, no explanation for that. I think 358/2 Days forced its narrative to retcon Xion in, so this just goes into the list of why I dislike this game.
As for why he can still dual wield, pretty sure those Keyblades are replicas based on his memories/data. Which is weird because he did have his Awakening. It could be one of them is real, but the other is a falsification. I'm going with that they are both fakes though. I stopped trying to make sense of KH's narrative fallacies.