This study investigates the influence of cylinder diameter and unequal external loading on the motion asymmetry of two independent double-acting pneumatic actuators. The analysis was motivated by rehabilitation-device applications in which two actuators are expected to perform simultaneous or symmetrical motion despite differences in external resistance. A reduced-order mathematical model was developed in MATLAB and included piston dynamics, nonlinear friction, variable chamber and tube volumes, isothermal pressure evolution, and compressible airflow through effective inlet and outlet restrictions. Two actuator configurations, with piston diameters of 32 and 40 mm and a nominal stroke of 0.4 m, were analysed under two experimentally derived unequal-loading conditions. The results demonstrated that load asymmetry produced measurable differences in piston-rod displacement, velocity, and stroke-completion time. Under the minimum unequal-load condition, both actuators completed the full stroke. The maximum displacement discrepancy reached 15.03 mm for the 32 mm cylinders and 12.88 mm for the 40 mm cylinders. Under the maximum unequal-load condition, the discrepancy increased to 91.73 mm and 71.02 mm, respectively, while the more heavily loaded actuator did not reach 99% of the nominal stroke within the 25 s simulation interval. Increasing the piston diameter from 32 to 40 mm reduced the maximum displacement discrepancy by approximately 14% under the minimum unequal-load condition and by approximately 23% under the maximum unequal-load condition. The results show that a larger piston diameter improves resistance to unequal loading but does not ensure synchronous motion. The developed model can support preliminary actuator selection and assessment of whether an additional pneumatic, mechanical, or electronic synchronization system is required.