One will be gauge sender the other EMU sensor. Get engine hot (warm) pull one and see if the gauge goes down. Then reconnect and try the other. Testing is a PIA but can be done with a good thermometer and a DVM (Using Ohms).
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i