Of course there are some causes which would cause a multiple engine out condition, poor fuel, volcanic ash etc.
However, statistically any engine is likely to suffer a malfunction induced failure before a multi engine out condition is caused by an environmental factor or an incident like the one that happened to Air Transat.
The fact remains that a single engine failure leading to an autorotation and water landing in the GOM is not actually that uncommon. Add some wind, no survival suits, winter, close to the end of daylight etc and you have several "escalation" factors, all of which could be mitigated by the use of twin engined helicopters.