I got it! I understand COMPLETELY! I think it's funny the other reviewers were confused, since it was so clear to me. The whole point is that one of them doesn't belong there. One of them isn't actually there...not in any real way. Or not in any normal way. It's almost like...like crossing a threshold in time. Like they were both there, in that hallway, on a Christmas Night, at one point in time...and somehow they were there together, despite the time that separated them.
It's a fascinating concept and I quite enjoyed reading it. I was a bit disappointed that the boy was James, though, since I'd hoped it was Harry...but it's okay. I suppose it couldn't be, since unless Harry was only a first or second year, he'd have recognized Tom. James is better for that reason, I suppose. All-in-all, a delightful read.