Mats attached to the same thermostat will heat to the same temperature. This is why it is recommended that every room have its own thermostat since every room has its own base temperature. It is also strongly recommended that areas with different types of flooring or subflooring be heated with different thermostats.
However, using one thermostat to control two or three mats in the same room is fine as long as the combined mats draw less than 15 amps of power and the room has the same flooring throughout.