It is quite normal for pilots to be rated on both the A320-series and A330 aircraft, and to be rostered to fly either or both in a given month. Whether BMI adopt this procedure, I can't say, but Monarch and others do. It is one of the major advantages (supposedly) of the Airbus Common Cockpit Qualification. We at Virgin used to have pilots rated and rostered on A320 and A340 at the same time, using the same CCQ. We no longer operate A320/321s, but we do operate A340-300 and -600 aircraft on the same roster.