I might wait till all the frames are drawn and 5 frames are solidly covered in brood.
Running two boxes is sensible if you are going to let the bees overwinter on their own honey. On just the one box, once you have taken the flow frames off for the winter where are the bees going to keep their stores?..unless, of course, you are going to rob every skerret and feed them sugar syrup.
Here in the UK I wouldn’t expect any honey from a nucleus colony in its first year.