> There is no way to introduce a new queen if one is already there.

That is what most people believe, and as a general rule, it is so close to true
that it can be considered to be true, as it undoubtedly is in the case
described.

Nonetheless there are exceptions.  Apparently laying queens can occasionally be
found naturally working side-by-side in hives.  Usually we think they are mother
and daughter, nonetheless, we do not know for certain how the second queen got
there.

Moreover, artificial two-queening is possible.  There are conditions under which
a number of laying queens can be successfully introduced into a single hive.
Usually the trick is to get them established and laying in separate hives or
completely separated parts of the hive and then place an excluder on one brood
chamber and set the other containing the second laying queen on top, maintaining
separation by means of the excluder.  (See the archives for more details).

Apparently in season and with nectar and pollen coming in, the excluder is
actually unnecessary, but AFAIK it is always used.  At the end of the season,
the excluder is pulled out and the belief is that one of the queens eventually
disappears.  People like to think it is the poorer one that perishes, but, then
again, we don't really know.  We are not even certain how often one perishes and
how often they both carry on, since most people find one queen in a hive and
quit looking.

I am sure that tolerance of several queens by bees is a function of many
factors, including season, locale, hive population and race of bee.  I have
often though that is the bees could be selected and bred to tolerate several
queens like the fire ants do, that this would reduce winter loss and increase
crops.

I have recently heard talk by some beekeepers that there is a 'secret' technique
whereby additional mated, laying queens are chased into hives in mid-spring and
apparently are accepted.  The results are stronger hives that produce more honey
with fewer problems.  I don't know if it is necessary to avoid use of excluders
above the brood boxes for this trick, but I suspect that if the queens have full
range of the hive that they are less likely to encounter one another.

Has anyone tried this?  It should be simple to find out if additional queens are
accepted by simply arranging a dead bee trap to be in place for several days
after chasing them in and checking it a few times starting shortly after
introduction.

allen
http://www.internode.net/HoneyBee/
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It's always darkest just before it gets pitch black.