Why would a diesel engine have 2 fuel pumps.?

Most diesels and some direct injection gas engine have two or three fuel pumps/injector pumps. The first pump...usually electrical and mounted in or near the fuel tank.. is used to draw fuel from the fuel tank to supply fuel to the filters and to the second pump...often a mechanical unit which can utilize engine power to produce much higher pressures... which bumps up the pressure immensely....some systems can run 15-30,000 psi or more. No need for that high pressure in the rest of the fuel system but you still need a way to transfer fuel from the tank to the mechanical pump or injection pump.. A separate injector pump may also be used to distribute and time proper fuel delivery to the injectors but as a separate unit, may need to be serviced less often or less expensively than a combo high pressure/injector pump or the high pressure pump is easier to locate on the engine block away from the injector pump.

Electric pump in tank feeds fuel at ~ 60 psi to mechanical pump on engine that feeds fuel to the direct injectors at ~ 1700 psi (maybe even higher). The need for high pressure is to inject fuel directly into a very high compression cylinder used in diesel engines. The key is direct injection straight into cylinder instead of port injection on many gas engine that spray gas into intake port which has vacuum or little pressure if turbo charged.

Edit: Injector pump is usually used just to prime pump and injectors if air has been allowed to get in system.

One's a main fuel pump and the other is a transfer pump if it has two tanks. The injection pump turns the 60 psi fuel pressure into 20,000 psi fuel pressure from the injection pump to the injectors.

Getting the diesel fuel to the cylinders so the motor runs!

You sure its they are fuel pumps and not oil pumps. The injector pump would be the fuel pump. The mechanical pump would be the main oil pump and the electric pump would be a backup for the mechanical pump/. That's my best guess.