They should be fine together. As long as there aren't adults with them, they should be fine. Just be sure they have plenty of space. That way there is less chance the smaller of the two will get stepped on.
When you put them together, watch them for a little bit. That way if the smaller one starts to get picked on you can separate them. But you should be good. I put week old ducklings in with my 3 week old ducklings and didn't have any problems : )