This paper presents a new thermodynamic closure formulation appropriate for the simulation of subcritical multiphase flows, where two liquid/vapor components need to be considered, in addition of an arbitrary number of perfect gas non-condensable components. Assuming each component to follow the Noble-Abel Stiffened Gas (NASG) equation of state allows to derive a fully explicit formulation for the mixture pressure. A matching phase transition solver is then proposed, to compute efficiently and accurately the thermochemical equilibrium of the said mixture. The model capabilities are then illustrated through a two-dimensional simulation of a liquid ammonia tank leak into 80%-saturated humid air, where water is found to significantly condensate when interacting with the ammonia jet.