  We study the impact of perturbative reheating on primordial observables in
models of multiple-field inflation. By performing a sudden decay calculation,
we derive analytic expressions for the local-type non-linearity parameter
$f_{\rm NL}^{\rm local}$, the scalar spectral index $n_\zeta$, and the
tensor-to-scalar ratio $r_T$ as functions of the decay rates of the
inflationary fields. We compare our analytic results to a fully numerical
classical field theory simulation, finding excellent agreement. We find that
the sensitivity of $f_{\rm NL}$, $n_\zeta$, and $r_T$ to the reheating phase
depends heavily on the underlying inflationary model. We quantify this
sensitivity, and discuss conditions that must be satisfied if observable
predictions are to be insensitive to the dynamics of reheating. We demonstrate
that upon completion of reheating, all observable quantities take values within
finite ranges, the limits of which are determined completely by the conditions
during inflation. Furthermore, fluctuations in both fields play an important
role in determining the full dependence of the observables on the dynamics of
reheating. By applying our formalism to two concrete examples, we demonstrate
that variations in $f_{\rm NL}$, $n_\zeta$, and $r_T$ caused by changes in
reheating dynamics are well within the sensitivity of Planck, and as such the
impact of reheating must be accounted for when making predictions for models of
multiple-field inflation. Our final expressions are very general, encompassing
a wide range of two-field inflationary models, including the standard curvaton
scenario. Our results allow a much more unified approach to studying two-field
inflation including the effects of perturbative reheating. As such, entire
classes of models can be studied together, allowing a more systematic approach
to gaining insight into the physics of the early universe through observation.
