>Have any of you experienced catching multiple queens in a swarm?
Often. It's very common. Here is one likely cause:
"On the fourteenth, the fifth young queen appeared, and the hive threw a swarm, with all the concomitant disorder before described. The agitation was so considerable, that a sufficient number of bees did not remain to guard the royal cells, and several of the imprisoned queens were thus enabled to make their escape. Three were in the cluster formed by the swarm, and other three remained in the hive. We removed those that had left the colony, to force the bees to return. They entered in hive, resumed their post around the royal cells, and maltreated the queen when a duel took place in the night of the fifteenth, in which one queen fell. We found her dead next morning before the hive; but three still remained, as one had been hatched during night. Next morning we saw a duel. Both combatants were extremely agitated, either with the desire of fighting, or the treatment of the bees, when they came near the royal cells. Their agitation quickly communicated to the rest of the bees, and at mid-day they departed impetuously with the two females. This the fifth swarm that had left the hive the thirtieth of May and fifteenth of June. On the fifteenth, a fifth swarm cast, which I shall give you no account of, as it showed nothing new."--Fran?ois Huber, New Observations on the Natural History Of Bees Volume I
http://www.bushfarms.com/huber.htm#letter9Another thing I have observed is that often two swarms hanging in the trees will merge over time as one shrinks and the other grows.