how to hook up 2 thermostats in series?
If you install two thermostats in series, they'll both have to be calling for heat for the heating to come on.

For example if thermostat 1 is set to 80, and thermostat 2 is set to 75. When the temperature drops below 78, thermostat 1 will close and call for heat. However, the heating will not come on.  When the temperature drops below 73, thermostat 2 will close and call for heat.  At this point the heating will come on, and will stay on until thermostat 2 is satisfied. 

In a situation like this, thermostat 1 will never be satisfied, and will always be calling for heat.  You'll basically be bypassing thermostat 1. So the temperature near thermostat 2 will be comfortable,  while the temperature near thermostat 1 will likely be somewhere less than 80.

Keep in mind, however, if the user of thermostat 1 figures out what you've done. They could turn their thermostat way down, and the heating would never come on.