This research proposes a method for the simultaneous acquisition of the second harmonic (2f) signal of quartz-enhanced photoacoustic spectroscopy (QEPAS) and the first harmonic (1f) signal of quartz-enhanced photothermal spectroscopy (QEPTS) based on the dual-quartz-enhanced photoacoustic–photothermal spectroscopy. The laser beam is first wavelength-modulated by the injection current and then intensity-modulated by an acoustic-optic modulator. The frequency of the wavelength modulation is half of the QTF1 resonant frequency, and the frequency of the intensity modulation is equal to the QTF2 resonant frequency. A modulated laser beam traveled through the two arms of the QTF1 and converged on the root of the QTF2. The 2f photoacoustic and 1f photothermal signals are concurrently obtained using the frequency division multiplexing technology and lock-in amplifiers, which allows the simultaneous detection of the gas concentration and laser light intensity. CH4 is chosen as the target gas, and the variations of the 2f photoacoustic and 1f photothermal signals are evaluated at various gas concentrations and light intensities. According to the experiments, the amplitude of the 1f photothermal signal has a good linear connection with light intensity (R2 = 0.998), which can be utilized to accurately revise the 2f photoacoustic signal while light intensity fluctuates. Over a wide range of concentrations, the normalized 2f photoacoustic signals exhibit an excellent linear response (R2 = 0.996). According to the Allan deviation analysis, the minimum detection limit for CH4 is 0.39 ppm when the integration time is 430 s. Compared with the light intensity correction using a photodetector for the QEPAS system, this approach offers a novel and effective light intensity correction method for concentration measurements employing 2f analysis. It also has the advantages of low cost and compact volume, especially for mid-infrared and terahertz systems.