I have run several 2-queen hives (vertical, not side-by-side).  I've tried both single deeps and double deeps for each each separated by a queen excluder with a second excluder under the supers.

With this system, the colonies build up much faster and, with the larger combined foraging force, the honey crop will be much larger.

Swarming was the primary reason why I discontinued this set-up.  No matter how many supers I put on in timely fashion, the 2 broodnests would be very crowded and plug up excessively with honey on good flows.  I believe, the excluders get perceived as barriers by the bees and they move the honey up into the supers at a slower rate causing the honey congestion in the broodnest which leads to swarming.

Frequent inspections and cutting out swarm cells was the only way to keep to bees from going up into the trees.  The extra honey seemed to make up for the extra work.  Another plus having extra queens on hand. 

I have abandoned this system for unlimited broodnests.  I get the same or better honey production with a single queen and no excluders.  The broodnest stretches into a nice, vertical oval across the 3 deeps and there is no need for an excluder to hinder the bees in the supers.  Broodnest congestion is much reduces.  Swarming onset is delayed if not prevented and it ends sooner.  No huge bee beards at the entrance.

Waldemar 

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