In serial femtosecond crystallography (SFX) experiments performed at X-ray Free Electron Lasers (XFELs), micron-sized crystals are typically delivered into the X-ray beam interaction region via liquid microjets. In this study, we numerically investigated a double flow-focusing nozzle, in which a primary fluid with sample crystals is surrounded by a secondary fluid, both being accelerated by a co-flowing sheath of gas through a converging capillary. Our numerical simulations consider water and ethanol as primary and secondary liquids, respectively, with helium as the focusing gas. Water and ethanol tend to mix during this micro-jetting process, influencing the material properties of the resulting jets. Previous numerical investigations of such flow configurations have assumed either non-mixing liquids or thoroughly mixed water and ethanol solutions already at the capillary inlet. The present novel model considers coupled mass, momentum, and concentration equations for a mixture-formulated two-phase (gas-liquid) and two-component (water-ethanol) Newtonian incompressible system. The density, viscosity, and surface tension non-linearly depend on the mixture concentration. The OpenFOAM FVM-VOF framework solves the model with an extended "interFoam" solver to account for the binary system. The resulting stability, diameter, length, and velocity of the jet are simulated as a function of the operating process parameters.