I once united 2- two story colonies in the fall. I placed one on top of
the other, and fed the colony for winter.That colony produced a medium
super of soft maple, at the end of a warm March. Both queens were
laying. I see colonies with two queens every season. I kept track one
year during re-queening. When finding the old queen, I shook all the
bees in the colony through an excluder. I checked 100 this way. 5 had
with two queens.
    One year, I re-queened by killing the old queen, and introducing a
new one with an introduction cage- all wire, fitting nicely between the
combs. I left the corks in the hole for 3 days. I then replaced the cork
with a marshmallow. In most cases, when I retrieved the cage to do this,
all was calm. Bees walking on the cage happily. Queen calm etc. Every
once in awhile I'd pull a cage and it would be covered with tightly
clinging bees, obviously enraged at this recent intruder. Every one of
those colonies had another "old" queen.