As Patrick says, the top brood box can become very heavy.
I have one hive on double brood, If I remember correctly it first ended up as a double brood when I united two colonies and then the colony rapidly expanded late in the season, each time I think I'm going to get it down to one box there's just too many bees.
However, I do not run it in the traditional sense of a double brood with the queen having access to both boxes. During the active season I restrict the queen to the bottom box with a Q/Ex between the two boxes. Each inspection I rotate 3 or 4 frames of brood from the bottom to the top box, with the frames taken from the top box providing laying space. I use one frame of drone comb in the bottom box which most of the time deters the bees from placing drone cells on any of the other frames (drones emerging in top box will go up and run a mock in the supers).
It's not unlike a Demaree, but with the advantage that the supers aren't between the two brood boxes, meaning you can check the supers or add additional supers without disturbing the colony. Also, you're not having to lift the top brood box from such a height, which I know I would struggle with.
The first time I rotated frames I did so because the colony had started to draw queen cells, I rotated some frames and added the Q/ex to see what would happen (sometimes we have to experiment), I checked a few days later, all queen cells gone and queen laying in expanded space in bottom box. I can't always get to inspect my bees at regular intervals, inspections are often a month apart, and find that using this method bides me some time and it has proved to be a good swarm prevention method for me.
For those who have trouble finding queens, at least you know which box she will be in.
I used to write a monthly article for our local association newsletter and when I described this method, a member contacted me to say they were going to give it ago because they were struggling trying to operate Demaree. Like all things bees, some will like it and some won't, in the end you have to do what works for you.