System and methods for processing a substrate using a reactor with multiple heating zones and control of said heating zones using a common terminal shared between two power supplies are provided. The reactor includes a heater assembly for supporting the substrate and a showerhead for supplying process gases into the reactor. An inner heater and an outer heater are integrated in the heater assembly. An inner power supply has a positive terminal connected to a first end of the inner heater and a negative terminal is connected to a second end of the inner heater that is coupled to a common terminal. An outer power supply has a positive terminal connected to a first end of the outer heater and a negative terminal connected to a second end of the outer heater that is coupled to the common terminal. A common-terminal heater module is configured to receive a measured temperature that is proximate to the inner heater. A desired temperature setting is received and a servo control law is processed to identify a direct control setting of an inner voltage of the inner power supply and an open-loop control setting of an outer voltage for the outer power supply. The outer voltage is defined as a ratio of the inner voltage.