Very few to my knowledge. The majority of members of my bee club are outraged that the City of San Diego demands requeening every 2 years.
As an aside, I agree with you that 2 brood boxes probably has zero impact on the swarming urge. I do it to ensure that the bees have sufficient supplies during a nectar dearth or over winter. We have pretty severe dearths, and I hate to feed if I can just ensure that they have extra supplies instead. I think the only real way way to prevent swarming is monitor and split when needed, and even that is not 100%.
