Our flight planning depatment run three scenarios from the ETP and the one requiring the most fuel is applied. The 3 scenarios are;
1/Engine failure and descent to appropriate height.
2/ Engine failure and depressurization with descent to 10,000ft
3/ Depressurization, two engines, descent to 10,000ft
Most of the time the 2eng depressurized is the most critical. The flight plan fuel is 'built up' to ensure that at the ETP there is enough fuel to cover the worst of these three scenarios.