Okay here are the results (done with the key in run, engine not started):
If you take a wire and short the two contacts together on the double prong unit, it sets some relay in the engine compartment but the gauge does not change.
If you run a wire from the lead going to the single prong unit to the chassis ground, it will cause the gauge to go to zero.
So the one you need to replace is the single prong one. (as do I, these used to cost 60 bucks, I think I will replace it now that it is reasonable.)