  This article extends the single-fluid relativistic irreversible
thermodynamics theory of {\it M{\"u}ller}, {\it Israel} and {\it Stewart}
(hereafter the {MIS} theory) to a multi-fluid system with inherent species
interactions. This is illustrated in a two-fluid toy-model where an effective
complex 4-velocity plays the role of a primary dynamical parameter. We find
that an observer who resides in the {\it real}-part of this universe will
notice that their knowledge of the universe parametrized using {\it real},
rather than {\it imaginary}, quantities are insufficient to fully determine
properties such as the total energy density, pressure or entropy, In fact, such
an observer will deduce the existence of some negative energy that affects the
expansion of their perceived {\it real} universe.
