  We demonstrate an easily tunable locking scheme for stabilizing frequency-sum
of two lasers on a two-photon ladder transition based on polarization rotation
in warm rubidium vapors induced by magnetic field and circularly polarized
drive field. Unprecedented tunability of the two-photon offset frequency is due
to strong splitting and shifting of magnetic states in external field. In our
experimental setup we achieve two-photon detuning of up to 700 MHz.
