The gravity fuel feed ceiling is based upon whether the fuel in the tanks has deaerated and this is related to flight time above FL300.If you have been above FL300 for ten minutes and both pumps fail, you could maintain your level and feed from the tank that has working pumps. Whilst this is taking place the fuel is deaerating . You can then use the entry in the gravity feed procedure >30 mins and maintain your level rather than have to descend to a lower level and burn more fuel. The problem wil be that you have created a fuel imbalance by doing so, and if it was excessive how do you correct it.
It is an odd scenario and not of my making!