Correct me if I'm wrong, but... I believe that when you're running a single engine, with any boost pump on, both pumps operate and provide 200% of what the engine needs into the fuel feed manifold. Then then excess fuel is returned to the tank of the operating side engine, via a return line and motive flow ejectors. This can easily cause an imbalance, but which is usually corrected with auto crossflow, when operating. I believe our MEL procedure when operating with an MEL'd crossflow pump is to operate with both engines running, so that the excess fuel returns to both wing tanks, instead of just one.