There are two engine driven pumps, two prime pumps, two transfer pumps and two non return valves, but the way I see it if one of those valves sticks open on a switched off transfer pump (normal practice), then the other transfer pump will continue circulating fuel back into the main tank instead of the supply tanks. As the transfer pumps warning lights work on amps drawn, it would not illuminate!
Imagine the scenario. Pilot knows how much fuel should be remaining backed up by main tank fuel gauge. If as the AIB suggested there is 95 lt fuel remaining then in the cruise it is likely the rear transfer pump would run light and so be switched off. Forward pump would still be operating normally so no warnings there. But if the aft check valve is stuck open, that fuel is not getting to the supply tanks. Pilot gets low fuel warning but is puzzled because he knows there is fuel in there, he knows his transfer pump is working, does he believe that warning light? First engine flames out. Pilot has 2 or 3 mins to sort this out or land. If he can visualize his fuel system and tanks, he may just pull the nose up and get some fuel over the rear fence, turn on his rear transfer pump, anything to get some fuel in the supply tanks. Pumps cavitating, engine trying to re light.
All conjecture I know, but it's my best guess for a double engine flame out and stopped drive train. Also I do know they were doing extensive fuel system testing on a 135 next to our hangar on Friday!