The temp sensor locations make the dual temperature settings on the thermostat tricky on our Duo-Therm 15,000 BTU units. The rear sensor is close to the air duct and cools quickly. The front sensor is on the driver side above the side window and is easily heated by sunshine and ambient air influencing the reading to the thermostat.
So, I set the thermostat for the rear to a much colder than desired temp to keep the AC on long enough to reach the desired temp and the front thermostat to a higher than desired reading to allow the AC to cycle at a comfortable temperature.