We report the measured production rates of unstable isotopes 9Li and 8He produced by cosmic muon spallation on 12C using two identical detectors of the RENO experiment. Their β-decays accompanied by a neutron make a serious contribution to backgrounds of the inverse beta decay events in precise determination of the smallest neutrino mixing angle. The mean muon energy of its near (far) detector with an overburden of 120 (450) m.w.e. is estimated as 33.1 ± 2.3 (73.6 ± 4.4) GeV. Based on 3100 days of data, the cosmogenic production rate of 9Li(8He) isotope is measured to be 44.2 ± 3.1 (10.6 ± 7.4) per day at near detector and 10.0 ± 1.1 (2.1 ± 1.5) per day at far detector. This corresponds to yields of 9Li (8He), 4.80 ± 0.36 (1.15 ± 0.81) and 9.9 ± 1.1 (2.1 ± 1.5) at near and far detectors, respectively, in a unit of 10−8μ−1g−1cm2. Combining the measured 9Li yields with other available underground measurements, an excellent power-law relationship of the yield with respect to the mean muon energy is found to have an exponent of α = 0.76 ± 0.05.