Can you explain what do you mean by waiting for fuel to deareate? If you are asking how to gravity cross feed on two engines the answer would be you can't. With fuel pumps on one side failing you would apply FUEL L (R) TANK PUMP1+2 LOW PRESSURE procedure. Much before you create such an imbalance you should be applying WHEN TK(affected) FUEL RQRD: which would lead you to gravity feed on that side.