On the first 2 stress tests you needed multiple bnets to get multiple access because it was under a single "classic beta", like the one "ptr"

On the last stress test they tested closer to what the final setup is going to be and if you notice you should have "World of Warcraft Classic" in your "Version" dropdown and the Account drop down is separate, so you were able and you will be able to use any account under 1 bnet (or more) that has an active sub