Alpha players get the beta builds approximately 2 weeks before released to beta. This is just so they can iron out the worst problems before unleashing it to a larger user base.
I can understand how this will work for the first Beta as there is no Beta yet. Going forward when Beta is live, how can they run two different versions of Beta along side each other unless they are on different servers? I would (guess) that Beta may be taken down for a few hours with access then limited to Alpha's to 'test' before being released to the masses. This would make more sense than running different versions of Beta for 2 weeks.
Please please please correct me if I have got this all wrong.