w124 Dual Fuel Pump Question

[79300D]

After having an interrmittent problem with my 1995 E320, I have a question regarding the dual pump system. One pump was making a really loud noise (whistling, whining ), and, I had absolutley NO power over 2500 rpm. Below 2500, car worked fine. After not driving the car for a few days, now, no noise, and the car is working great. Obviously this is a short term abhoration. Okay, my question is.. how do I now know which fuel pump to replace? Do both pumps run continuously, or, when needed?

Thanks

Jeff