The optimization of the optical scheme design of a mode-mismatched dual-beam thermal-lens spectrometer for differential (dual-cell) measurements in a far-field mode using diffraction thermal-lens theory is carried out. A criterion for an expert estimation of the quality of the spectrometer design for differential thermal-lens measurements in analytical chemistry (sensitivity, low limits of detection, and quantification) is also developed. The theoretical calculations agree well with previous papers on differential thermal lensing. Using the example of iron(II) tris-(1,10-phenanthrolinate), it is shown that the blank signal compensation in differential thermal lens spectrometry provides a decrease in the limit of detection by an order of magnitude compared to the decrease in single-cell measurements. Using an artificial two-component mixture of ferroin and potassium dichromate, it is shown that dual-beam differential thermal lens spectrometry makes it possible to determine trace components against 900-fold excess amounts of interfering substances.