In this perspective, mixed ionic liquid acidity, basicity and dipolarity/polarizability are discussed in terms of Kamlet-Taft and Catalan polarity scales. Properties and experimentally-derived Henry's constants of mixed ionic liquids with CO2 are summarized. Then challenges and knowledge gaps are discussed in the areas of solvent polarity, solvent properties and solvent systems. The mixture of two ionic liquids with different functionalities combined with supercritical CO2 can provide a versatile biphasic solvent system for processing biomass and biomass-related compounds.