  Two-photon emission of an electron in an electromagnetic plane wave of
vanishing frequency is calculated. The unpolarised probability is split into a
two-step process, which is shown to be exactly equal to an integration over
polarised subprocesses, and a one-step process, which is found to be dominant
over the formation length. The assumptions of neglecting spin and simultaneous
emission, commonly used in numerical simulations, are discussed in light of
these results.
