A numerical study was carried out using an in-house vortex filament code named DR HAWT (Dual-Rotor Horizontal-Axis Wind Turbine code), to identify non-dimensional parameters for dual-rotor wind turbines (DRWTs). DR_HAWT was verified and validated using various blade-vortex interaction cases as well as the NREL Phase VI and MEXICO experiments. Three dimensionless parameters (diameter ratio, gap ratio, and combined tip-speed ratio) were derived and tested using DRWT models based on the NREL Phase VI rotor geometry and operating conditions. Further studies suggest an improved performance of DRWT at lower diameter and gap ratios, as well as a reduction in angle of attack (AOA) along the inboard section of the downwind blade. These studies also revealed that the NREL Phase VI rotor equipped with a 5-m-diameter upwind rotor extracted more energy than two equivalent single-rotors at 7.5 CTSR, resulting from a shift in the operating range due to the variation in AOA.