Hi Neil
This is something that always gets beekeepers chatting - do you use a single BB, Brood and a half (BB + 1 super) or 2 BB, and the merits for and against.
I have used all three, but not because I had an 'Italian' bee - just because I wanted to try them out. A double BB can be handy if your queen is prolific and needs space to lay (a colony in it's 2nd/3rd yr onwards). It can therefore possibly slow down the urge to swarm.
If swarm cells are built, they are generally built along the bottom of the combs of the upper BB - so you can just tip the BB to 50 degrees, smoke and then expose and cells and manage accordingly.
Having a double BB can be handy if you are managing using 2 BB systems, or looking for increase - take one BB away with cell in, leave old one on site with Q.
That said, many commercial beekeepers only manage on single broodboxes - Maisemore who run over 1000 hives just use a national BB, and they do alright (but time, expense and weight is a factor here)
Read up on double brood systems - great fun for the hobbyist, Ron Brown mentions it in his book 'A seasonal Guide'.
regards
S