Steady natural convection induced by multiple discrete heat sources (DHSs) in two-dimensional horizontal enclosures is numerically investigated. A general combined temperature scale method is first implemented to identify the DHSs of both external and internal types. An associated concept of thermal strength is then introduced in terms of the unified heat transfer characteristics of DHSs to analyze the interaction among them. Four different calculation cases are detailed analyzed, and main attention has been focused on the effects of the Rayleigh number (Ra), the thermal strength (Pi), and the separation distance (W*) on the interaction between DHSs. Computational results demonstrated that the combined temperature scale method and the unified heat transfer characteristics analysis are convenient and efficient to evaluate the complex interaction between DHSs and its effects on the fluid flow and heat transfer structures in horizontal natural convection enclosures.