We describe a new alexandrite laser source arrangement designed to measure atmospheric water vapor using the differential absorption lidar technique. This laser is capable of emitting two pulses at two appropriately selected wavelengths within a single flash lamp discharge. A narrow spectral linewidth of Deltalambda < 1 pm is obtained for each pulse by intracavity filtering with a birefringent filter and two Fabry-Perot interferometers. Wavelength commutation between the two pulses is performed by electro-optically tuning the birefringent filter. The temporal separation between the two pulses can be chosen between 50 and 70 micros and each pulse duration is <250-ns (full width at half-maximum). Typical output energies of 50 mJ/pulse at each wavelength are obtained with this laser system at a 10-Hz repetition rate for a 1.3-kW input electrical power.