A theoretical investigation of the dynamic sensitivity of a two-frequency laser detector is presented in which the laser is also used as the source of sounding radiation. It is shown that, under conditions determined in the paper, the dynamic sensitivity of the two-frequency laser detector can substantially exceed the sensitivity of the single-frequency one. Based on the theoretical analysis and the experimental results of Ref.t9, a highly sensitive lidar scheme with a two-frequency laser detector is proposed. In this scheme the ensing is performed with the strong beam, while the signal is detected with the weak beam, generated at a transition coupled with the sounding beam. It is found that in this sounding regime nonlinear stabilization of the sounding beam intensity takes place. It is also noted that in addition to the highly sensitive regime there also exists a regime of two-frequency laser emission in which the laser detector sensitivity can decrease even below that of single-frequency laser detection.