Measurement and quantification of fission products such as 134Cs, 137Cs, and 154Eu can be used to determine burnup of used nuclear fuel by measuring the intensity of the gamma signals emitted via gamma-ray spectroscopy. However, spent nuclear fuel is comprised of several other fission products that produce a large gamma background, which makes it hard to accurately isolate gamma signals of interest (604.72 keV line from 134Cs) from the background signal from these other fission products (especially the 661.66 keV line from 137Cs). One of the techniques that could solve this issue is by applying the γ-γ coincidence technique to remove uncorrelated gamma ray noise from the time correlated gamma signals from 134Cs. In this study, two CLLBC scintillation detectors with good resolution (~3.5% at 661.66 keV) were used to acquire LIST mode gamma-ray spectra from a complex gamma source containing 134Cs, 137Cs, 60Co, and 154Eu. CLLBC detectors have reasonably good energy resolution, are portable (potentially useable in an underwater safeguards instrument), and have the ability to simultaneously measuring neutron counts using pulse shape discrimination. The LIST mode data acquired was processed with a γ-γ coincidence algorithm to eliminate the uncorrelated background counts. The signal-to-background ratio in the prominent gamma peaks of the fission product isotope 134Cs are compared between singles and coincidence spectra. This work demonstrated the concept of using γ-γ coincidence counting to quantitatively measure the 134Cs content in a mixed gamma source using CLLBC detectors.