  The pattern of calculation of amplitudes of a series of processes in the
field of an intensive laser wave, in which two fermions $(p; p')$ and two real
photons $(k_1; k_2)$ participate, is considered. In relation to one-photon
processes, these processes are of the second order on $\alpha$, if the wave
intensity $\xi \ll 1$ (i.e., actually absorption from the wave only one
quantum). Otherwise, they are competing and essentially nonlinear. One-photon
processes have a number of the important physical applications. For example,
${\gamma}e$ and ${\gamma}{\gamma}$ colliders work on their basis.
  In DSB the calculation is conducted at the level of reaction amplitudes. It
essentially simplifies both the calculation and the form of obtained results;
those combinations of amplitudes which describe the spin effects are easy to
calculate. And these effects are especially essential in nonlinear processes.
The calculations are conducted in covariant form. Besides compactness, this
provides independence of the frames of reference and energy modes. The masses
of fermions are taken into account precisely and are not assumed equal each
other (heavy leptons, modes of various decays). The pattern of calculation is
given in the channel indicated in the title. The amplitudes of this channel are
also applicable for calculating of induced decay of orthopositronium into two
photons. And in this approach there is no necessity to assume the relative
momentum equal to zero.
