MFF
I spent 2.5 years flying A320/A330 mixed fleet.
It's surprisingly straightforeward. Biggest issue is the size difference and you need to think about your wingtip clearance and mainwheel location while on the ground.
Now I fly A330/A340 mixed fleet. Biggest issue is the huge difference in performance between the two types. System architecture is the same.
I've also flown 757/767 on a common type rating.
Airbus is well ahead of Boeing in terms of commonality between different aircraft types.