In the present study a new energy theorem is proposed, "The two actionstheorem", which is valid in linear elastostatic problems. A new formalism concerning the works done by the external actions is introduced. Known energy theorems are proved using the proposed two-actions theorem. A composite materials problem is confronted in terms of the two actions theorem and energy relations are formulated. Finally, it is presented a study on the problems of a composed two material hollow cylinder under internal and external pressure, and of stretching of an infinite plate with an inserted elastic disc of a different material. The proposed energy relations are verified in these applications.