The one on the drivers side is the AC fan and the one on the passenger side is the radiator fan. Each fan has a separate fuse and relay under the hood. Generally both fans should be running if the AC is on or the engine temperature switch is calling for cooling. Both fans pull air through the AC condener and engine radiator.
I would guess the two fans are for redundancy, in case one fan dies you still have at least 50% air flow.
Also the radiator has a rectangular shape so two smaller fans cover more area than one large fan.