  Self-seeding schemes, consisting of two undulators with a monochromator in
between, aim to reduce the bandwidth of SASE X-ray FELs. We recently proposed a
new method of monochromatization exploiting a single crystal in
Bragg-transmission geometry for self-seeding in the hard X-ray range. A
straightforward extension is to use such kind of monochromator setup with two
-or more- crystals arranged in a series to spectrally filter the SASE radiation
at two closely-spaced wavelengths within the FEL gain band. This allows for the
production of doublet -or multiplet- spectral lines. Applications involve any
process with a large change in cross section over a narrow wavelength range, as
in multiple wavelength anomalous diffraction. Here we consider the simultaneous
operation of the LCLS hard X-ray FEL at two closely spaced wavelengths. We
present simulation results for the LCLS baseline, yielding fully coherent
radiation shared between two longitudinal modes. Mode spacing can be easily
tuned within the FEL gain band. The XFEL output intensity contains an
oscillating "mode-beat" component whose frequency is related to the frequency
difference between the pair of longitudinal modes considered. At saturation one
obtains FEL-induced modulations of energy loss and energy spread in the
electron bunch at optical frequency, which can be converted into density
modulation at the same optical frequency using a weak chicane installed behind
the baseline undulator. Powerful coherent radiation can then be generated with
the help of an optical transition radiation (OTR) station. We also consider how
the doublet structure of the XFEL generation spectra can be monitored by an
optical spectrometer. The OTR coherent radiation pulse is naturally
synchronized with the X-ray pulses, and can be used for timing the XFEL to high
power conventional lasers with femtosecond accuracy for pump-probe
applications.
