Oh I thought you meant: Allow voting after two hours have passed. But you meant: Allow voting until two hours have passed (?).

If people cannot upvote top comments, then the problems mentioned in my reply to this comment should apply: https://news.ycombinator.com/item?id=5430699

Also, forbidding votes on top comments doesn't help a comment at position 10 or 20 to surface to the top. (So really useful comments posted rather late, would still be forgotten forever)