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The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard S-NPP provides global coverage once per day for the reflective solar bands. With the successful launch of NOAA-20, the global coverage of VIIRS has now been doubled. In order to use NOAA-20 VIIRS data for environmental related studies, the radiometric performance of VIIRS needs to be independently validated. In addition, for long term studies that use data from multiple satellites instruments, it is imperative to have radiometrically accurate and consistent data products from all the instruments. This study uses SNO (Simultaneous Nadir Overpass) and extended SNO over Saharan desert to assess the radiometric performance of NOAA-20 VIIRS relative to S-NPP VIIRS. Since direct SNO doesn't exist between NOAA-20 and S-NPP, the study uses MODIS as a transfer radiometer. Both NOAA-20 and S-NPP have SNOs with MODIS. Double differencing technique is used to estimate the radiometric bias between the two VIIRS instruments. The study suggests that NOAA-20 VIIRS reflective solar bands are consistently lower in reflectance than that from S-NPP VIIRS by about 2%. Larger bias is observed for bands M5 (0.67 mu m) and M7 (0.86 mu m) bands mainly because S-NPP VIIRS absolute calibration for these bands is biased high by about 2%. The impact on bias due to spectral differences between the two VIIRS instruments is quantified using hyperspectral measurements from Sciamachy.