Temperature sensors, why two of them

Engine Computers always use a two-wire temperature sensor, whether for liquid or air. That is because the ground return circuit is monitored by the computer even though it is shared with other sensors. Typically you'll find 0.2 volts on that ground wire. It goes back to the computer, then through it before going to ground on one of the computer's ground wires.

Dash gauges use a single-wire sensor. That sensor is the last leg in the gauge's ground circuit.