Actually I respectfully correct you...Roxas was able to use two keyblades because of Ventus's heart as well as Sora's heart because during KH1 Ventus was still there.
Also Roxas looks the way he does now because Ventus was in Sora's heart before Sora used the keyblade of hearts to separate Kairi from him. There, it created his heartless, and being that Ventus was inside Sora's heart that's the reason why Roxas (Sora's nobody) looks the way he does now in the Kingdom hearts series.
Everything I said takes place before even Roxas existed, KH1 and Birth by Sleep play a big role on the topic.
And being that some people hadn't beat all the games and followed the story extremely carefully (like I have because I love this series with all my heart and yes I own two of the DS games 358/2 days and Re:coded...) then you'll probably understand everything yourself.
