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good Afternoon Stephen, I am Kevin....
You are talking about the thermostat, the T5900, and state that you have two of them, are they both connected to the same air handler?
Ok, just a statement, if you set your thermostat at 75, and it doesn't cool to 75 degrees, setting it to 72 will not make it any colder. The thermostat is only a switch to turn on and off the heat or cooling cycle. I think you need to look at the unit which is not cooling. the simple things you can look at are the air filter, is the outdoor unit running as well? Each indoor unit has an outdoor condensing unit unless it is geothermal or self contained.
let me know what you find out there.