Typically the queen will start using the 2nd deep when the bottom one is full. Normally you would add the 2nd deep when there were only 1 or 2 frames not in use in the bottom one. Now you may have a bit of a problem if they have half filled the 2nd deep with sugar water nectar. Hopefully when the current capped brood hatches she will re-lay in the bottom and also the top brood frames and they will start clearing the sugar water out of the non brood frames in the top. You might want to consider stop feeding them sugar and adding a honey super on top.
I'm trying to go the other way. Had a 3 deep hive which swarmed and left a 100 lbs plus of capped honey in the the top deep and a honey super, with brood and honey in deeps 1 and 2. I've consolidated the brood down to the bottom deep, removed approx 70 lbs of capped honey frames from the top deep and the honey super and would really like them to move some of those stores from deep 3 down to deep 2 so I could retrieve the 3rd brood box and use it elsewhere,