Although a large portion of the flight was spent over the Atlantic Ocean, when crossing to/from North America, there were alternative airports available within two hours. The Azores and Iceland are frequently used as alternate divert destinations.
It is extremely uncommon for both engines to malfunction. Only one such occurrence comes to mind. Air Transat 236 was an Airbus A330 in August 2001 that suffered a significant fuel leak.
All of the fuel in the aircraft was burned or leaked through a hole in a high-pressure fuel line because it was difficult to figure out what was going on.
Consequently, the lack of fuel caused both engines to fail. The airplane landed in the Azores.
Modern airplanes are built with multiple backups so that if an engine or system fails, a safe landing can still be accomplished.
