The knowledge of mixed gas solubility in ionic liquids at high pressures, which becomes relevant in applications such as tertiary oil recovery and landfill gas utilization, is critical for the design of gas separation technologies. In this study, we examine ionic liquid mixtures for tuning the solubility of carbon dioxide (CO2) and methane (CH4). Using Gibbs ensemble Monte Carlo simulations, absorption isotherms of pure CO2 and CH4 are computed in the binary ionic liquid mixtures containing the common cation 1-n-butyl-3-methylimidazolium [C(4)mim](+) and the anions chloride Cl- and bis(trifluoromethanesulfonyl)imide [NTf2](-) using five molar compositions at 353 K and the pressures ranging from 1 to 100 bar. Additionally, mixture gas solubilities of CO2 and CH4 with the starting gas phase mole ratios of 5:95 and 15:85 at a total pressure of 100 bar are determined and compared with the ideal predictions obtained from the solubilities calculated in pure ionic liquids. Results show that the mixture solubilities deviate from the ideal mixing rule at high pressures with improved solubilities in ionic liquid mixtures having up to 10% of [C(4)mim][NTf2] in [C(4)mim]Cl. Furthermore, for the entire ionic liquid mixture composition range, the CO2/CH4 solubility selectivities exhibited a nonlinear behavior. Interestingly, simulated CO2/CH4 solubility selectivities at the mole ratios of 05:95 and 15:85 do not differ significantly from the ideal solubility selectivities suggesting independent gas absorption, except for 05:95 at 10% of [C(4)mim][NTf2] in [C(4)mim]Cl. Thus, there is a potential for improving CO2/CH4 selectivity in pure [C(4)mim]Cl with the addition of a small amount of [C(4)mim][NTf2] ionic liquid.