  The usual paradigm of open quantum systems falls short when the environment
is actually coupled to additional fields or components that drive it out of
equilibrium. Here we explore the simplest such scenario, by considering a two
level system coupled to a first thermal reservoir that in turn couples to a
second thermal bath at a different temperature. We derive a master equation
description for the system and show that, in this situation, the dynamics can
be especially rich. In particular, we observe prethermalization, a transitory
phenomenon in which the system initially approaches thermal equilibrium with
respect to the first reservoir, but after a longer time converges to the
thermal state dictated by the temperature of the second environment. Using
analytical arguments and numerical simulations, we analyze the occurrence of
this phenomenon, and how it depends on temperatures and coupling strengths. The
phenomenology gets even richer if the system is placed between two such
non-equilibrium environments. In this case, the energy current through the
system may exhibit transient features and even switch direction, before the
system eventually reaches a non-equilibrium steady state.
