  In this work, by considering an isentropic fluid-fluid interaction model with
a large symmetric drag force, a commonly used simplified two-fluids flow model
is justified as the asymptotic limit. Equations for each fluid component with
an interaction term are identified in addition to the simplified two-fluids
flow model, which can be used to resolve the density of one fluid specie based
on information on density and the velocity of the other fluid specie, i.e., the
testing flow.
