If 2 BBs are allowed to be built, then 2 BBs should be allowed to fly at the same time. If only 1 BB is allowed to fly, then the player with the second BB logs in to the server and is told he isnt allowed to play. This hapenned on the last server with DNs. If that player wants to fly, he then has to sell his ship, then buy it back it later, in the meantime waiting for a new ship. A playter who only has maybe an hour or so to log in and play has just lost a significant percentage of his playing time.
Basically its just a hassle having to do this and for no good reason. If only 1 BB is allowed to fly at any given time, then only 1 BB should be allowed to be built.