Thats true, they should be able to portray the voice the way they see fit to the scenario
-I still have the Voice from BTAS playing over and over in my head and come to think of it Owens voice would probably match up well with Two-Faces. But if you saw Inside man\ King Arthur\Sin City you can notice that his emotions arent expressed so well with his voice at least thats the way I saw it (especially when he was wearing a mask in Inside Man), and hes never really altered that his tone of voice for a character hes got the same tone in all his roles. Norton has kind of a weasel like voice and I cant see that as Two Face, but I think Nortons Voice acting is better than most so you never know, and unlike Owen hes changed his style of voice acting before (The score\Primal Fear\American History X) but then his regular tone of voice that you can make out in most his roles are more like what he sounds like in Italian Job, Fight Club and 25th hour. Im an animator so I think voice acting is pretty much the most important key to the success of the character but especially with a character like two face youre going to have two different personalities and two different voices and so far Owen hasnt really altered his voice for any of his roles but that doesnt mean that he cant do it.