Yes the gas tank will start emptying when you take the upper fuel pump off, be careful.

You theoritically don't have to change both pumps but if one died the other one is on its death bed.

You probably could check them outside of the car. It would be fairly easy to check them for flow, harder to check them for pressure. When I changed mine they both, ran but one rattled It acted up when it was warm so I couldn't really check for that