As an engineer i always found the 777 very reliable (last worked on it 8 yeras ago) other than a few software resets compared to the Airbus which throws a wobbly and can only be recovered by complete down powers, albeit on the ground.
The Trent engines other than a few sensors and thrust reverser defects are very reliable.
I would imagine the investigation will lean towards computer glitches rather than fuel contamination...for both wing tanks to be contaminated to exactly the same extent and cause simultaneous shutdowns seems vey unlikely or though a possibilty... the amount of water found in the 330 wings during weekly fuel/water drains shows exactly how much fuel/water the engines can deal with.
Well done to the pilots.
P.s the way i remember the 777, with no PFC's they were only flying on 2 spoilers and the stabiliser which if this was the case makes the landing even more remarkable.