Then simply setup the second BES server to use the same config database. That way the user will be unaware that they were moved to another BES. A single config database is key to maintaining and administering a multiple BES environment.
Unless of course you are using local MSDE, but then again if you can afford multiple BES servers you can surely pay for a SQL license.
This single database would also help if one BES fails (with mission critical users), then they can be easily moved over to another available BES server.