Splitting up heat in 2 unit

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Hi everybody I am wondering if anyone would have any advice/experience for splitting up the heat in a 2 unit property. Currently, the property has one central gas(forced hot air) furnace in the basement that heats the upstairs and downstairs apartments. There is only one thermostat controlling the heat. (Upstairs apartment controls the temperature for downstairs apartment) I am looking for any advice as to what route to go splitting up the heat for this building. Thanks!!