Hey Benet,
The reason I'm pro 2 donks is not just due to engine failures it's all the other bells and whistles that twins come with. Such as redundant electrical and hydraulic systems. Stability systems and sometimes coupled auto-pilots. Full instrument packages with an ADI that isn't between your legs and that is big enough to fly with for hours on end. Rad alts and instantaneous VSIs.
I work in a different environment then maybe you do with having the city of London or similar around you. I fly in a very dark place with very little to no ground lighting for extended periods in much of my call area. I mean no lights not even one for 30-45 minutes. I would imagine over Puget sound it would have been pretty dark as well.
My guess is that the Agusta A109 didn't go down due to lack of an engine but more likely some other factor like CFIT or some other more chatastrophic emergency than an engine failure. I don't fly Agusta A109's but I would hazard a guess that you can maintain altitude in cruise on a single engine.
Anything that can be done to make things safer for us makes it safer for our passengers and that's who were there for in the first place.