Anthill,
The failure of both engines is the scenario I was thinking of with the equivalent disaster scenario being the failure of 3 engines on a 4 engine jet. I know the following is very much simplified and exaggerated but if you assume your outsourced maintenance provider starts giving you dud engines periodically such that on average they fail one in every thousand flights. With twin engines one flight in a millions ends in catastrophe whereas for a four engine jet I think it would be one flight in every 250 million.