Cardiac single-photon emission computed tomography (SPECT) using two radioisotopes, technetium-99m (Tc-99m) and thallium-201 (Tl-201), has wide clinical application due to its diagnostic advantages. One of the main problems in simultaneous dual-isotope imaging (SDI) is the increase in the contribution of scattered photons due to the difference in the energy of photons emitted from the two radioisotopes, resulting in a decrease in image contrast, as well as an incorrect estimate of the activity distribution. In this study, in the first step, in order to determine the optimal main energy window, the scatter fraction was evaluated in the windows of 25%-30% with energy centers of 68-80 keV for Tl-201 radioisotope and the windows of 15% and 20% with energy center of 140 keV for Tc-99m radioisotope. In the next step, by setting a secondary energy window (with a width of 22 keV) in the Compton region of the energy spectrum and two narrow energy windows (with a same width of 4 keV) on both sides of the optimal main window, the performance of the dual-energy window (DEW) and triple-energy window (TEW) correction methods in estimating the scatter contribution of the main energy window and improving image quality in both simultaneous dual-isotope imaging and separate single-isotope imaging methods was evaluated and compared. The results of this study show that the use of DEW and TEW correction methods leads to improved image contrast and signal-to-noise ratio for both Tl-201 and Tc-99m radioisotopes in both simultaneous and separate imaging methods. Also, the relative increase in contrast for both radioisotopes in the simultaneous method is greater than that in the separate method, and for the radioisotope Tl-201, it is greater than that in Tc-99m.