Future Air Traffic Management is expected to shift towards four dimensional trajectory (4DT) management, requiring new decision support tools for air traffic controllers to meet stringent time and position constraints. In previous work, a prototype human-machine interface has been developed for 4D trajectory manipulations of single aircraft. This paper describes a tool for multi-aircraft manipulation and investigates its potential control efficiency benefits. A human-in-the-loop experiment (N = 13) has been conducted using scenarios with sector disruptions and varying conflict geometry. Results show that participants preferred to use multi-aircraft manipulation for groups of aircraft having small convergence angles. Since the current implementation involves re-routing all selected aircraft through one common waypoint (referred to as a 'merge point'), extra additional track miles were flown and airspace robustness reduces. Regarding efficiency and safety, multi-aircraft trajectory manipulation seems favourable only for smaller convergence angles, although this also depends on the way the operators place the aircraft merging points. For future development, attention should be devoted to making flight efficiency constraints of each aircraft more salient, enabling controllers to better time the rerouting multiple aircraft and more fairly distribute re-routing costs.