Several kinds of vehicles are so complex and require such multitasking that they cannot be operated by a single person: tanks, trains, certain airplanes, etc.
Take the typical tank, for example. If you're in the driver's seat, you have limited visibility and you can't load or fire the main gun. If you're in the commanders seat, you can see all around you, but you can't drive, shoot or load the [...] [tvtropes.org]