Influence of structure difference of the two layers on epidemic spreading. Panels (a) and (b) show the case of β₂ = 0, where and represent the infected densities of pathogen-1 at the up and down layers, respectively. The parameter β₁ is chosen as β₁ = 0.003 > β_1c0 in panel (a) and β₁ = 0.0014 < β_1c0 in panel (b). Panels (c) and (d) show the case of β₁ = 0, where and represent the infected densities of pathogen-2 at the up and down layers, respectively. The parameter β₂ is chosen as β₂ = 0.005 > β_2c0 in panel (c) and β₂ = 0.003 < β_2c0 in panel (d).