Well what happens is that the old queen goes with the swarm, not the new queen. This is what surprised me but the reason why is because there can't be two queens in a hive else they will attack each other until one is dead. The old queen leaves before the new queen is born. I believe there agree a number of queen cross layed to give more of a chance of the old hive having a queen but if there's a problem with all of them, the hive is done for.
Kind of odd but also makes good sense.
There are exceptions to everything though! In some hives where "supercedure" happens , an old queen and new queen can be side by side on a comb, both laying. Unusual but it happens apparently. In general though, two fertile females in the home is never a good idea