The optical properties of multi-metal nanocomposites made from Cobalt (Co), and Silver (Ag) are analyzed theoretically within the mean field approximation, and experimentally verified using absorption spectroscopy. The experimental system was modeled as a thin layer composed of hemispherical nanoparticles formed by grains of Co and Ag in contact with air and the SiO substrate. The main aspects of the absorption curve, such as the shape and spectral location of the localized surface plasmon resonance, are well captured by a simple self-consistent mixing model using a modified Maxwell-Garnett approach and the Milton lower bound.