Gentlemen and ladies,
Various scenarios have been posed on this thread that involve some type of fuel contamination. Whatever caused the two engines to misbehave, the only evidence we have so far indicates that one was stricken only 8 seconds before the other, and that their malfunctions were similar.
Although it is theoretically possible for a common-cause fuel contamination to develop to a critical point in two separate tanks at the same moment, even after a long flight and a complete descent, reason suggests this is improbable.
Therefore, these scenarios are all predicated on the assumption that, at the very least, the fuel crossfeed must have been open at the time of these failures.
I know of no evidence – so far – that this was the case.