  We isolate the two-step mechanism involving a real intermediate photon from
the one-step mechanism involving a virtual photon for the trident process in a
constant crossed field. The two-step process is shown to agree with an
integration over polarised sub-processes. At low to moderate quantum
non-linearity parameter, the one-step process is found to be suppressed. When
the parameter is large, the two decay channels are comparable if the field
dimensions are not much greater than the formation length.
