In this paper we report the investigation of a beam coupling effect that occurs during the formation (by UV curing) of POLICRYPS diffraction gratings. Along with observations, we present a complete theoretical model which accounts for the main experimental features of the effect. Numerical solutions confirm the observed absence of any energy transfer process for unit ratio of the impinging beam intensities. When this ratio is not unit, the transfer is instead present and tends to equalize the beam intensities during the curing process; the capability of performing this equalization is strongly related to the grating diffraction efficiency. Furthermore, in this case, numerical simulations enable also to visualize the final (distorted) morphology of the fabricated structure.