We have studied the alpha-decays of Po-214 into Pb-210 and of Po-212 into Pb-208 tagged by the coincidence with the preceding beta-decays from Bi-214 and Bi-212, respectively. The Rn-222, Th-232, and Rn-220 sources used were sealed inside quartz vials and inserted in the Counting Test Facility at the underground Gran Sasso National Laboratory in Italy. We find that the mean lifetime of Po-214 is (236.00 +/- 0.42(stat) +/- 0.15(syst)) mu s and that of Po-212 is (425.1 +/- 0.9(stat) +/- 1.2(syst)) ns. Our results, obtained from data with signal-to-background ratio larger than 1000, reduce the overall uncertainties and are compatible with previous measurements.