A double brood box is needed in colder climates to add to the ‘body heat’ in the colony to keep the cluster warm, but what I do in what I suspect a very similar climate is to use a single brood box and if that hive is getting too strong I make a split to make another hive and so produce more honey. I’m now building past 20 hives and aiming for 50 as there is a big demand for my honey. I make a $ from selling full complete hives and from honey. When I say complete hives I mean a full working apiary hive, brood box and super producing honey. there are guys about me selling nucs and even swarms but I have no trouble selling a complete hive to those that want to have honey sooner rather than later: if that makes sense.
For me making a split and a second hive produces more honey than a single “super hive”, but you may have to wait for the splits to really get booming again. Bee keeping is a lot of forward planning and a bit of wishful thinking as well. 
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