Understanding dual fuel pump workings

[884201972]

I have an '88 gas 420sel with I think fuel issues. I have pulled the fuel tank and tank screen. Both are beautifully clean. My car has dual fuel pumps, but I do not understand how they work. The smaller line that goes to the fuel filter and then pump does not run gas out of it when disconnected. The larger line from the bottom of the tank just pours gas when disconnected from lines on fuel pump. But this line does not go through fuel filter, the smaller line does but does not allow gas to pour out when disconnected. Does it require fuel pump to be running to "pump gas"?
Why does this car have dual fuel pumps when earlier models appear to have only one?
The issue I was having when running was car will not go above about 25 miles per hour, as it then starts bucking like starving for fuel.
Before this issue, it was running great, and fuel pump relay went out. I jumpered connections and car ran again, but did not drive it far. Now with new pump relay, I am having the fuel starving issue. As a note, the new relay does not have all the pins the original does, but I was assured by shop that I think knows these cars well (mercedes their specialty) this would not make a difference. Does the extra pins run the second fuel pump and I am only running on one pump now? This is getting old real quick! Thanks if someone can explain the two pump issue and give me some insight. Steve