Can I add a slave to existing thermostat?
Placing a second parallel thermostat or moving the single thermostat to the colder room will not be an ideal solution for you. The reason being is that the second room will call for more heat and indeed warm up but the master room will get overheated. 

The proper way to fix this is to re-balance the heat registers (or duct deflectors if they are present) to allow less warm air to enter the master room and proportionally more to enter the second room.