This work aims to investigate the accuracy of quantitative SPECT imaging of 177Lu in the presence of 90Y, which occurs in dual-isotope radiopharmaceutical therapy (RPT) involving both isotopes. We used the GATE Monte Carlo simulation toolkit to conduct a phantom study, simulating spheres filled with 177Lu and 90Y placed in a cylindrical water phantom that was also filled with activity of both radionuclides. We simulated multiple phantom configurations and activity combinations by varying the location of the spheres, the concentrations of 177Lu and 90Y in the spheres, and the amount of background activity. We investigated two different scatter window widths to be used with triple energy window (TEW) scatter correction. We also created multiple realizations of each configuration to improve our assessment, leading to a total of 540 simulations. Each configuration was imaged using a simulated Siemens SPECT camera. The projections were reconstructed using the standard 3D OSEM algorithm, and errors associated with 177Lu activity quantification and contrast-to-noise ratios (CNRs) were determined. In all configurations, the quantification error was within ± 6% of the no-90Y case, and we found that quantitative accuracy may slightly improve when 90Y is present because of reduction of errors associated with TEW scatter correction. The CNRs were not significantly impacted by the presence of 90Y, but they were increased when a wider scatter window width was used for TEW scatter correction. The width of the scatter windows made a small but statistically significant difference of 1-2% on the recovered 177Lu activity. Based on these results, we can conclude that activity quantification of 177Lu and lesion detectability is not degraded by the presence of 90Y.