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Is this a Coleman thermostat and if so, which one? If you pull the cover off there should be a part number. Or attach a photo of it to your post. Has anyone done any work on either A/C lately or did this just start? Get back to me with this information and I can assist you further here. All follow ups are included in our fee.
I can't answer this without knowing the make and model of both A/C units and the make and part number or model of the thermostat. Too many variables here without knowing all the facts first. Get back to me with this and I can make some suggestions on the problem and solution.