  We investigate the two-photon decay rate of a highly excited atomic state
which can decay to bound states of lower energy via cascade processes. We show
that a naive treatment of the process, based on the introduction of
phenomenological decay rates for the intermediate, resonant states, leads to
lower-order terms which need to be subtracted in order to obtain the coherent
two-photon correction to the decay rate. The sum of the lower-order terms is
exactly equal to the one-photon decay rate of the initial state, provided the
naive two-photon decay rates are summed over all available two-photon channels.
A quantum electrodynamics (QED) treatment of the problem leads to an
"automatic" subtraction of the lower-order terms.
