  The hydrodynamic description of a superfluid is usually based on a two-fluid
picture. We compute the basic properties of the relativistic two-fluid system
from the underlying microscopic physics of a relativistic \varphi^4 complex
scalar field theory. We work at nonzero but small temperature and weak
coupling, and we neglect dissipation. We clarify the relationship between
different formulations of the two-fluid model, and how they are parameterized
in terms of partly redundant current and momentum 4-vectors. As an application,
we compute the velocities of first and second sound at small temperatures and
in the presence of a superflow. While our results are of a very general nature,
we also comment on their interpretation as a step towards the hydrodynamics of
the color-flavor locked state of quark matter, which, in particular in the
presence of kaon condensation, appears to be a complicated multi-component
fluid.
