As far as I am aware both Monarch Airlines and Airtours International in the UK operate Mixed Fleet Flying (Cross Crew Qualification or CCQ). The CCQ rating exists for their Airbus pilots so that they can fly A320/A321 and A330 types simultaneously.
I am not sure what proportion of their Airbus pilots hold this qualification, perhaps all? I'm pretty sure all their Airbus pilots can fly A320 & A321 as I belive this is the same type rating anyway and so is automatic. My guess is there is an hours requirement before they have the A330 added though, and I understand this requires a seperate ground school course (7 days or so). Maybe a stint in the sim too?
Any MON/AIH guys care to shed some light?