Diesels have two batteries 90+% of the time. Glow plugs take alot of current, and extra help for the starter with outrageous engine compression. (17:1 or so). Some passenger car diesels have only one batt, but every pickup I have seen has duals.
The second alt is optional. Usually for work trucks/service trucks that may want an arc welder run off of the engine. Also, some types of service requires it. Ambulances to name one.