Cloudy prediction for plane parallel slabs with total log(N_H/cm⁻²) = 20.5 and n_H = 0.5 cm⁻³, illuminated by the quasar pair QSO1 and QSO2, placed at a separation of 2.9 Mpc, as derived from their systemic redshifts. Top left: column density of H I, N_{H I}, as a function of distance. The data-points are color-coded by the ionization parameter. In this scenario most of the models are optically thick to the ionizing radiation, N_{H I} ≫ 10^17.2 cm⁻², with only the CGM regions of the two quasars characterized by optically thin gas. Top right: predicted SB_Lyα for the Cloudy models without (solid blue line) and with (dotted blue line) the UVB. The red dotted line indicates the total SB_Lyα, summing up the Cloudy prediction and the scattering contribution estimated following Sect. 5.2.2. The gray shaded region shows the range of observed SB_Lyα. Bottom left: predicted He II/Lyα ratio as function of the distance from the quasars without (solid blue line) and with (dotted blue line) the UVB. Bottom right: predicted C IV/Lyα ratio as function the distance from the quasars without (solid blue line) and with (dotted blue line) the UVB. In each of the bottom panels, the dotted red lines represent the ratios corrected for the presence of Lyα scattering as modeled in Sect. 5.2.2, while the green horizontal line indicates the local 2σ upper limit on each ratio (Fig. 10). In all four panels, the vertical dotted lines indicate the position of the two quasars, while the striped yellow regions show the zones used to normalize the quasars PSFs.