In using lidar to measure quantitatively the extinction and backscatter coefficients in an inhomogeneous atmosphere, a second equation is required to invert the lidar signal. This additional information is obtained when a second lidar system is added, pointing in the opposite direction with respect to the first. From the returns of the two lidar systems both the spatial extinction and backscatter coefficients can be retrieved, supplemented by the normally missing interval close to the lidar system. The proposed method requires two calibrated lidar systems and assumes the single scattering model.