I second the caution:
Swarming is a complex set of variables. Adding another brood box is not the simple do-all fix. More brood boxes means more brood space, which means more bees, a soon to be bigger hive, which may just result in bigger swarm and multicasting. I am not in Ozzland, but just as fyi I do run doubles for parts of the season and singles for other parts of the season. Depends on the goals for that particular hive and the signals the bees and the environment are telling me.
There are two entities involved in swarms. 1-The Queen , 2-The Bees. The beekeeper has to be aware of each of their seperate needs and attentive to the current conditions in the hive. So as to look after the separate needs of both of them, at the right time, in order to control the hive and avoid problems.
Bee-"keeper" is exactly that. Keeping the bees in the boxes. When a swarm leaves, you are not a beekeeper anymore. You are demoted to a beehadder, or beehavver if there are enough of them left behind to watch come and go. When you make it through a whole next season without losing any, then you may be promoted to beekeeper again.