By almost 4 orders of magnitude, solar irradiance is the primary source of natural forcing operating on the Earth’s atmosphere. There are two critical aspects to the measurement of solar irradiance: 1) measuring the solar irradiance traceable to international radiometric standards, and 2) understanding and correcting long-term instrument degradation for the determination of full-spectral solar variability. The Total Solar Irradiance Sensor (TSIS) on the International Space Station measures spectral irradiance with the Spectral Irradiance Monitor (SIM). This was launched in 2017 and was built on heritage and lessons-learned from the SORCE-SIM (Solar Radiation and Climate Experiment), a satellite which was launched in 2003. Almost 2 years of overlapping observations were collected from both instruments before the SORCE mission ended in February 2020. We present the analysis of the solar irradiances as measured by both instruments throughout the overlap period in which we quantify the wavelength dependent long-term stability. Moreover, we provide a late mission absolute flux correction for SORCE-SIM which was launched with an absolute irradiance scale less accurate than TSIS by almost a factor of 10. This will help to ensure the continuous data record of solar variability is maintained and kept to the required degree of accuracy, and will therefore help to lower uncertainties when constructing composite data sets.