I don't think losing Ven's heart will do anything to Sora.
Look at him from between when he turned into a Heartless in KH1, and the end of CoM. He didn't have Ven's heart for that entire time and he was fine!
Only thing will be dual wielding, but you know what? It's possible to dual wield with one blade. That's how Roxas and Sora operated until Sora went to sleep in CoM; they were sharing one blade between the two of them. If Sora learns how to control that somehow, perhaps he could dual wield XD
Look at him from between when he turned into a Heartless in KH1, and the end of CoM. He didn't have Ven's heart for that entire time and he was fine!
Only thing will be dual wielding, but you know what? It's possible to dual wield with one blade. That's how Roxas and Sora operated until Sora went to sleep in CoM; they were sharing one blade between the two of them. If Sora learns how to control that somehow, perhaps he could dual wield XD