I thought some people liked secrecy when they voted. That will be gone if they have to vote for a certain candidate on a certain day. And what if you're not voting for either of them? If I were the type to vote, my vote would be for Rick Astley.
Also, would it even matter? People will be out voting on one day or the other anyway. Half of them Obama and half of the McCain, theoretically. So no matter what, wouldn't you have half of them on one day and the other half on the other either way? What if 90% of America is voting for McCain? Then one of those days is going to be really fucked.