Some reasons for recommending two deep brood boxes:
- Swarm prevention
- Population expansion
- Common bee-space issues
- Use of Queen excluders
- No slatted rack
Weight & Inspections: I don't disagree, necessarily, with two brood boxes but I guess my main concern is the weight of lifting a second deep box will limit the beekeeper from inspection responsibilities when the reasons for keeping two deep brood boxes may be using some logic in a situation that does not apply to their colonies.
Population: In the northeast U.S.A. colonies do not have a long enough nectar and pollen flow (5.5 months) to build up large numbers as they do in the Southern U.S. We may be talking 40,000 vs 80,000 bees at peak population time in mid July. If your source for recommending two brood boxes comes from a southern beekeeper, this may be their logic. The queens in the south may need to utilize two deep boxes for brood.
Make sure, if you do stack two deep brood boxes on top of each other that the bee space, 5/16 inch, is either on the top or the bottom and the same on each box. Bee space is sometimes accommodated on both the top of the frame bars and below the frames as well by the manufacturer as a buffer between the bottom board and the brood frames. There's a solution to that dilemma below.
Queen Excluders - If you are using a queen excluder, which is not that common in a northern climate, it would be constraining your brood nest area to one box. I use 8 frame equipment and do not use an excluder and I do have brood up in the supers above during the season. I don't mind harvesting honey from frames formerly filled with brood. Honey is naturally anti-microbial and the 1st job the newly born bee has is to clean her own cell. Beeswax is anti-microbial as well as is propolis. If this is a problem, the Queen excluder is for you. Drones can get stuck and die in the excluder openings, however. I found that to be too much for me on the season I practiced with one, so be prepared.
Honey Excluders - Up here in the NE the comment is often made by the old time beekeepers, that queen excluders are more like honey excluders. If your colony came from a breeder who routinely uses one the girls may be just fine with it. A queen usually does not cross evaporating or stored honey in a hive unless she's making a run for it. (Smoke at the entrance before an inspection can drive this behavior if you should fear she is up there and vulnerable to being exposed or lost.)
If you need to keep your queen safe - and you do if you are working with young people or students in your apiary or have the part time help of non-beekeepers or use a lot of smoke - a queen excluder can be a great tool.
Forager Traffic: What I do depend on to keep my brood box up out of the traffic lanes created by foragers coming and going or intake bees waiting for them to come in for a landing, and especially as a rainy day hangouts for adult bees, guards and foragers... is... drum-roll... A slatted rack!
What's A Slatted Rack? - you say! Why these great components are not praised more often bewilders me. They've been around for over 100 years invented by C.C. Miller and I first learned about them from Richard Bonney's book "Hive Management". They are simply a little "airplane hanger" for the adult flying bees and act as a buffer between the daily comings and goings and the brood area. My bees never build wax in them but will smear propolis on them as they will the frames. See more here.
That's my take on it and I have learned to never close the door on an idea so, as always, insights are certainly welcome. Looking forward to spring!