The reason is that there is a gas fireplace(stove) in the living room that has the pilot on all year. Currently the thermostat is in the hall keeping the back of the house cool. The heat from the fireplace pilot keeps the living room too warm so I want to control the ac unit from the living room in the summer and the heat from the hall in the winter. I am using nest thermostats
I'd suggest looking for a thermostat that supports remote sensors. I personally own ecobees which support up to 64 wireless sensors per unit. You can decide which of the sensors participate in measuring the average temperature for each time slot of the day. I believe Honeywell offers thermostats that support sensors as well in both the wired and wireless variety.
You could use a simple and cheap single double pole switch. In 1 position the system would use the hall thermostat in the other position the system would use the living room thermostat. The switch can be a very inexpensive one (spdt) with the common going to the furnace and the NO normally open to one. Then the NC normally closed to the other thermostat the system would chose the thermostat based on the switch position. If you have a higher end system a double pole switch may be required with the heat on one pole and the cooling on the other pole but both wired the same with NO, NC and common going back to the furnace / AC control.