  The observational signatures of multi-field inflation will generally evolve
as the Universe reheats. We introduce a general analytic formalism for tracking
this evolution through perturbative reheating, applicable to two field models
with arbitrary separable potentials. The various transitions, including the
onset of scalar field oscillations and the reheating of each field, can happen
in different orders and on arbitrary hypersurfaces. The effective equations of
state of the oscillating fields are also arbitrary. Nevertheless, our results
are surprisingly simple. Our formalism encapsulates and generalises a huge
range of previous calculations including two-field inflation, spectator models,
the inhomogeneous end of inflation scenario and numerous generalised curvaton
scenarios.
