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This study experimentally investigates the aerodynamic effects of rotor–rotor interaction in a twin-rotor system operating in ground effect at a rotor-tip Reynolds number of $10^5$ . The strength of the ground effect and the rotor interaction were controlled by adjusting the normalised ground standoff distance and rotor separation distance, respectively. For the single-rotor configuration, ground proximity generated a stagnation region within the wake, redirecting axial momentum radially outward to form a wall jet. As the rotor approached the ground, the stagnation region moved closer to the rotor disk, increasing the thrust coefficient. In the widely spaced twin-rotor case, the opposing wall jets from both rotors converged on the ground to form a stagnation point. From this point, the flow diverged outward, producing a fountain flow and transverse outflow. The fountain flow tilted the wakes toward each other, reducing thrust. As rotor spacing decreased, rotor-disk blockage intensified, suppressing the fountain flow. When the fountain-driven recirculating flow developed around the rotor tips, re-ingestion into the rotors caused substantial thrust reduction. Peak thrust loss could be identified using the momentum flux coefficient of the fountain flow. However, with very close rotor spacing, the weakened fountain flow contracted the recirculating region, suppressing wake deflection and largely restoring thrust. Importantly, the thrust loss induced by rotor interaction reached its maximum at smaller normalised rotor separation distances as the rotors operated closer to the ground. These findings quantitatively link the fountain-flow dynamics to thrust variation, offering new mechanistic insight into multirotor aerodynamics in ground effect.