I agree with Beth.
I have not do this either....
Keep in mind that if you ask 3 beekeepers the same question, you will
get 5 differet answers.
A few things I have heard about this technique.
You want the stronger hive on top, this is the queen that you want to keep (is this the new requeened one that swarmed?). If you don't want to
kill a queen, they will battle and the stonger one will win out or----
you may get lucky and the two will accept eachother because their smell has integrated and therefore you will have a two queen hive that builds up fast.
If you want to kill a queen - smush her on a low tree branch away from your hives, you may catch a new swarm.