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This paper treats the crane scheduling in a container port where two cooperative gantry cranes (denoted as twin cranes) jointly store import containers arriving from the seaside in a storage yard. We aim to minimize the makespan while non-crossing constraints among cranes need to be considered and preemptive container moves are allowed, i.e., the seaside crane sets down a container in some intermediate position where the landside crane takes over and delivers the container to its final storage position in the yard. Elementary complexity proofs are provided and efficient heuristic solution procedures are introduced and tested.