Why the thermal capacity of two mixed system is equal to the sum of individual thermal capacities?

Why the thermal capacity of two mixed system is equal to the sum of individual thermal capacities?

Let thermal capacity to be the specific heat multiplied by the mass of the thermodynamic system.

It is a experimental fact? Or there is some fundamentl principle guaranteeing this?

• For alloys, the Neumann-Kopp "rule" is often invoked, where the heat capacity of a phase is equal to the weighted average of the individual elemental heat capacities. However, this is not a rule or a law, just an often (but not always!) useful value to use if you have not explicitly measured the heat capacity. Many alloys violate this "rule". – Jon Custer Sep 29 '16 at 20:02