  We provide an analytical study of the coupling of short and long wavelength
fluctuation modes during the initial phase of reheating in two field models
like hybrid inflation. In these models, there is - at linear order in
perturbation theory - an instability in the entropy modes of cosmological
perturbations which, if not cut off, could lead to curvature fluctuations which
exceed the current observational values. Here, we demonstrate that the
back-reaction of short wavelength fluctuations is too weak to lead to a
truncation of the instability for the long wavelength modes on time scales
comparable to the typical instability time scale of the long wavelength entropy
modes. Hence, unless there are other mechanisms which truncate the instability,
then in models such as hybrid inflation the curvature perturbations produced
during reheating on scales of current observational interest may be very
important.
