Air Conditioning - installing thermostats

I have a situation where I would like to install two thermostats for the central air conditioner/heater one upstairs and the other downstairs. I would like to do this to see what is the optimum spot cause I am going to be installing a small ac unit upstairs that might effect the central ac and I am not sure if I want it downstairs or upstairs yet. In winter I don't know either so I was going to just install one up and the other down and see what works the best. I don't really know how to do this though. There can only really be one hooked up at a time sending a signal to turn the unit on and off, I think. I was thinking of some kind of a switch or something that directs power to one of them at a time. What do you think.

1 T-stat per unit or 1-Tstat per system.
The only solution for 2 locations is for 1 location to monitor the temp.

I have seen this done by adding a single pole - double throw (spdt) switch into the power (red) wire. Power would feed the switch & then on to either the upstairs or downstairs thermostat depending on the position of the switch.