I haven't seen the FSM, but I have been told by several independent sources that one sensor is for the gauge, whereas the other water temp sensor is just a temperature switch directly related to the glow plug duration. I could be wrong, but I do know first hand you cant switch them around and have your gauge work. 
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1985 MBZ 300D (DD, 240k, unmuffled hellion!)
1982 MBZ 300D (372k, retired for parts)
1981 MBZ 300D (575k, parts pig)
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