  We report on Doppler-free laser spectroscopy in a Cs vapor cell using a
dual-frequency laser system tuned on the Cs D$_1$ line. Using
counter-propagating beams with crossed linear polarizations, an original
sign-reversal of the usual saturated absorption dip and large increase in
Doppler-free atomic absorption is observed. This phenomenon is explained by
coherent population trapping (CPT) effects. The impact of laser intensity and
light polarization on absorption profiles is reported in both single-frequency
and dual-frequency regimes. In the latter, frequency stabilization of two diode
lasers was performed, yielding a beat-note fractional frequency stability at
the level of $3 \times 10^{-12}$ at 1 s averaging time. These performances are
about an order of magnitude better than those obtained using a conventional
single-frequency saturated absorption scheme.
