As much as I dislike the DC-10 historically, it's a nice idea to travel on something which you might never get the opportunity to do so again. The 777 and A340 will be around for the next 30-40 years. Probably the very same ones. You may be students now, but you'll both be retired before the 777 and A340 is phased out of service.
I know nothng of AOM other than it is a charter line based in France. It's probably a good charter line. The French don't tolerate substandard entities as many other nations do. They're usually pretty fussy.
As far as the twin-engine thing goes, make your wife understand that airliners don't crash because an engine dies. They crash for a myriad of reasons, but not because they lose engine power. Engine power is not required to fly. It's only needed to get you to your destination in time. In your case, engines simply get you to France. Without all the engines working, you must land before you get to France, or you go on to France, but at a lower altitude. But you will land either way.
Much better things to fear are:
-explosive fuel tanks
-faulty cargo doors
-worn elevator screwjacks
The full list is quite long and terrifying. But engines aren't on the list.
An optimist robs himself of the joy of being pleasantly surprised