  In this paper, we present the hydrodynamic limit of a multiscale system
describing the dynamics of two populations of agents with alignment
interactions and the effect of an internal variable. It consists of a kinetic
equation coupled with an Euler-type equation inspired by the thermomechanical
Cucker--Smale (TCS) model. We propose a novel drag force for the fluid-particle
interaction reminiscent of Stokes' law. Whilst the macroscopic species is
regarded as a self-organized background fluid that affects the kinetic species,
the latter is assumed sparse and does not affect the macroscopic dynamics. We
propose two hyperbolic scalings, in terms of a strong and weak relaxation
regime of the internal variable towards the background population. Under each
regime, we prove the rigorous hydrodynamic limit towards a coupled system
composed of two Euler-type equations. Inertial effects of momentum and internal
variable in the kinetic species disappear for strong relaxation, whereas a
nontrivial dynamics for the internal variable appears for weak relaxation. Our
analysis covers both the case of Lipschitz and weakly singular influence
functions
