In serial femtosecond crystallography (SFX) at X-ray Free Electron Lasers, micron-sized sample crystals are delivered commonly via liquid jets produced by gas dynamic virtual nozzles. In these systems, a co-flowing gas focuses a liquid stream from a feeding capillary. SFX experiments typically occur in low-vacuum environments, where compressible gas flows often reach supersonic speeds. This study numerically examines a double flow-focusing nozzle (DFFN), where a primary fluid carrying sample crystals is focused and accelerated by a miscible secondary liquid and high-speed gas sheath. In particular, it incorporates the critical influence of gas compressibility and temperature variations on primary-secondary mixing dynamics. The primary and secondary liquids are water and ethanol, respectively, while helium is the focusing gas. The mixing interactions between water and ethanol during the jetting process influence the material properties of the resulting jets. In contrast to earlier studies, which assumed either non-mixing liquids, pre-mixed solutions, or mixing under isothermal conditions, this research adopts a compressible two-phase (gas-liquid) and two-component (water-ethanol) framework. The numerical model includes coupled mass, momentum, energy, and species concentration equations. The variation of involved thermophysical properties such as density, viscosity, and surface tension is considered based on local mixture compositions with the non-linear Jyouban-Acree model. The simulations were performed using an extended version of the "compressibleInterFoam" solver within the OpenFOAM framework, enabling the modelling of compressible flows and binary fluid interactions. This advanced model offers a powerful numerical tool for analysing and optimising DFFN designs under high-speed gas flow conditions. Insights gained from this study will help enhance the nozzle designs for SFX experiments with improved jet stability, reduced sample consumption, and maximum jet length and jet velocity.