Ben XIII

It's not that hard to explain, really: Sora touching Ven's heart had an effect similar to the Keyblade Inheritance Ceremony - instead of him touching Ven's Keyblade, their hearts touched, and presumably since a Keyblade is so closely connected to the wielder's heart, that counted as touching it, hence giving him the ability to weild a Keyblade - ABILITY TO, mind you, not an actual Keyblade. He got his actual blade from Riku, as the latter had already lost his heart to the Darkness, and it became Sora's. Once Riku turned away from the Darkness, another Keyblade appeared to him to replace the one he had lost to Sora. Sora, no matter how strong his heart is, can only wield one Keyblade. However, since Ven's heart is within him as well, he can use Ven's blade as well as his own, allowing him to dual-wield. Since Roxas is essentially an extension of Sora, he can use Sora's Keyblade, and by extension, Ven's. I have a theory as to how dual-wielding will be implemented after Ven is saved: It is stated/implied (I can't remember if they actually SAY it) in Dream Drop Distance that Nobodies grow new hearts. Now, Roxas became more his own person than a part of Sora, so if any Nobody could grow a heart, it would be him. And Roxas is still within Sora. Since two hearts = two Keyblades, this could explain keeping dual-wielding even after Ven's heart is no longer resting within Sora. ... although I probably shouldn't talk, since I have this idea of Sora wielding the X-Blade at the end of Kingdom Hearts III which would be awesome, but probably wouldn't happen .-.