I have a two story house in Arizona. I have one unit for downstairs and one unit for upstairs. The unit upstairs has one thermostat to control all 4 rooms. The thermostat is located in common area where there is no duct but has 3 returns. This area is usually hotter than rooms. When I set the temp at a certain level, the master bedroom is very cold and other two rooms are warmer.

Is there a way I can have two thermostats for upstairs? One in master which control only master area and one inside one of the rooms so that rooms get better cooling? What kind of money am I looking to spend for this?

Is it a bad design that they put thermostat in the area with returns? Should it have been elsewhere?