  We theoretically study the relaxation of electron orbital states of a double
quantum dot system due to two-phonon processes. In particular, we calculate how
the relaxation rates depend on the separation distance between the quantum
dots, the strength of quantum dot confinement, and the lattice temperature.
Enhancement of the rates by specific inter-dot distances and lattice
temperatures, and the relative strength of different scattering channels are
discussed. Our results show that although at low temperatures ($T \sim 1$ K)
two-phonon processes are almost four orders of magnitude weaker compared to
one-phonon processes in relaxing electron orbital states, at room temperature
they are as important as one-phonon processes.
