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This paper presents a macroscopic bilateral modeling approach for Reconfigurable Intelligent Surfaces (RIS) capable of simulating various cases, including transmissive and bilaterally reradiating RIS. Building on recent work, the approach is based on a general power balance at the RIS surface according to a few parameters, that accounts for both desired and parasitic reradiation modes on both sides of the surface as well as diffuse scattering and dissipation. The surface effect is described using a spatial modulation coefficient that embodies the wavefront transformation of each reradiation mode, including phase and amplitude modulation. Here the model is proposed and combined with a Huygens-based field calculation method to test its effectiveness in a few applications cases found in the literature.