Speculating :
Perhaps the most likely cause of a double engine failure would not be a mechanical fault with the engines,
but loss of fuel - i.e running out of ( useable ) fuel, causing them go very quiet.
This
could be attributed to pilot error, poor maintenance, or possibly even mechanical failure of some sort,
perhaps even a multiple bird strike in the right ( wrong ) place, but none of these are restricted to twin engine aircraft,
and are just as likely / unlikely in a single - probably even more so in some cases.
I can ony think of a couple of examples of twin failures -
The Dyfed-Powys 109 G-DPPH that ran out of useable fuel in 2001 following maintenance to fuel pumps,
and I seem to recall there was an incident back in the '80's / 90's ( I think ) where a Police twin squirrel
( or maybe even a 902 ) lost fuel supply because of detached fuel line(s) -
resulting in an auto and run on landing into a field. Can't find any details on that one though.
