Feh, I think the comparison between Two-Face and Jekyll/Hyde is misguided. If you go back to the source material, you have to remember that Jekyll specifically chose to become Hyde, and continued to make that choice throughout the book until it became something he couldn't control. Harvey Dent never gets that choice - his scarring is forced upon him by sheer bad luck, and he isn't simply able to go from being two-Face back to being Dent again.
Really, Harvey has much more in common with Dorian Gray. While Jekyll has to make a transition to become Hyde, Dorian's 'evil self' is always present at the same time as Dorian, if locked in an attic, just as Dent's is always present on his face. Similarly, you'll notice that as time has gone by, artists have continually drawn Two-Face uglier and uglier as audience tastes grow...