H.R.
I think it would be best to have them together while they are younger because I know that young children always wonder where their presents are when the other is opening theirs. They aren't old enough to understand that is their own special day yet. And it would keep all the guest from having to make time for 2 occassions in a little over a week. And since they are about the same age you can have one theme that fits both their interests.