This is also the same situation in the A300-600 which controls which tank feeds the engines using the auto fuel feed system. Heres a quick look.
Should the aircraft have its center tank serviced with fuel prior to take-off, this would be the normal sequence of fuel feed (in the auto feed mode). The center tank initially feeds both engines, 3 minutes after engine start the center tank pumps stop and the LH and RH inner pumps turn on, feeding #1 & 2 engines respectively. After take-off slat retraction the center tank will again feed the engines until the tank runs dry, then the RH & LH inner pumps again turn on and feed their respective engine. When the inner tanks run dry the RH & LH outer tank will then feed their respective engine. Note: the outer tank pumps are not part of the auto fuel feed system so their pumps are always running. You may ask, if the outer tank pumps are always running, why doesn't the outer tanks deplete first? Its because the output pressure of the outer tank pumps are less than the inner and center tank pumps therefore the outer tanks will feed only if the inner and center tanks aren't.
Now back to the question at hand. The A300-600 has a tank in the stabilizer called the trim tank. This tank is mainly used for controlling the center of gravity by pumping fuel from the inner tanks and center tank to the trim tank. When the a/c decends to 20500 feet all the fuel in the trim tank is pumped forward to the center tank (not the inner tanks). Now at this point the inner tanks might be dry and the fuel remaining is in the outer tanks and the center tank. The center tank is now supplying both engines, but as soon as the slats are deployed the center tank pumps will shut off making the outer tanks supply its respective engine in essense trapping usable fuel in the center tank. What happens if the plane is in a holding pattern with its slats down, the crew watching his outer tanks deplete with fuel in the center tank not not being pumped? Simple, take the fuel system out of auto feed into manual feed (by deselecting certain pumps) and then he can pump out the center tank with the slats down.
The reason aircraft use the tank to engine configuration during take-off and landing (slats down) is so a failure of one system to supply fuel won't affect more than 1 engine during the critical phases of flight.
You're only as good as your last departure.