Move up a frame of brood into the center position in the top box.
Both upper nucs are in use, so it worked. As they hatch out more workers they should fill up the top box, I hope. By then they'll need a laying queen. If they don't requeen I'll combine them with another nuc that has a queen.I did that a few days ago. Will open the nucs in a few days and see if they took the hint.