>> 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

I agree with allen about exceptions to the rule.
Maybe the rules are wrong?  My observations are that mated queens are less
aggressive towards each other than virgins.
Mated queens in laying condition are even less aggressive.  Workers are also
less aggressive towards queens which are currently laying.  I usually
release mated laying queens from mating nucs direct into another colony with
out a queen cage, after removing the other queen. This will not work with a
mated queen that has been confined in a queen cage for a day or more.
When I cannot find the old queen I release the new queen into the hive.
This does not happen very often since I mark my queens.  Later I usually
find the new marked queen in the hive with the previous marked queen, both
laying.

Marc Studebaker
Geneva, IN