What do you mean by "such aircraft are fully certified to fly with one engine"? Following an engine failure, or engine shut down, the Airbus A330 has to land at the nearest suitable airport.
An engine failure on a twin engine aircraft is an emergency. The aircraft performance is highly degraded on one engine.
Oh, then I stand corrected. Perhaps I should have said "fully able to fly with one engine".
Many years ago, I was privileged to be part of a group that was given a ground tour of a 747. After walking past a 707, this thing looked HUGE ... and so did the engines.
Inside, we had a ground engineer escort us around and I had an opportunity to sit in the co-pilot's seat - and under the direction of the engineer, pushed some buttons! One of those brought up a display showing the air pressure in every one of the tyres. Very cool.
We then talked about the engines - and we were told that the 747 only needs the four engines to take off. So, yeah, it needs 4 engines. A fully laden Jumbo can fly and land on 2 - and an empty one on a single engine. So having these capabilities and knowing about them is essential in being able to assess a course of action in an emergency. They are not something you should plan on.
I asked what happens if, say, two engines failed on the same side of the aircraft - obviously curious on the resulting yaw moment. The reply came: "That's when the pilots earn their money".
It would seem very clear designing such capabilities into aircraft - especially passenger carrying commercial aircraft - has been around for a very long time. Further, inasmuch as much as we have had examples of aerial mechanics, that is not the norm. Problems need to be managed in the air since you can't just "pull over".