AIRBOEING's statement that the probable positions of the fuel shutoff valves:
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- the LP by the wing spar (generally placed well away from the engine so that it can do some good in case of fire),
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- the HP with the engine
supports the pilots' action to restore/continue the crossfeed -- fine as long as the LP shutoff does not fail and how do you catch that in time?
So, we may have either:
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- two component failures -- pipe and LP valve (with invidious implications for maintenance)
- pipe rupture above LP valve (less likely, but if so, a design shortcoming).
In either nasty case, a crossfeed valve is your last line of defense, as long as there's enough
accessible fuel for the remaining engine(s) to get to an airport.