Symmetric, target-directed, bimanual movements take less time to prepare than asymmetric movements (Diedrichsen et al. in Cerebral Cortex 16(12):1729–1738, 2006; Heuer and Klein in Psychol Res 70(4):229–244, 2006b). The preparation savings for symmetric movements may be related to the specification of symmetric amplitudes, target locations, or both. The goals of this study were to determine which symmetric movement parameters facilitate the preparation of bimanual movements and to compare the size of the facilitation for different parameters. Thirty participants performed bimanual reaching movements that varied in terms of the symmetry/asymmetry of starting locations, movement amplitudes, and target locations. Reaction time savings were examined by comparing movements that had one symmetric parameter (and two asymmetric parameters) to movements with all asymmetric parameters. We observed significant savings (~10 ms) for movements with symmetric amplitudes and movements with symmetric target locations. Reaction time costs were examined by comparing movements that had two asymmetric parameters (and one symmetric parameter) to movements with all symmetric parameters. We observed significant reaction time costs (~13 ms) for all movements with asymmetric amplitudes. These results suggest that movement preparation is facilitated when amplitudes or target locations are symmetric and that movement preparation suffers interference when amplitudes are asymmetric. The relative importance of the three parameters to movement preparation, from most to least important, is movement amplitudes, target locations, and then starting locations. Interference with asymmetric amplitudes or target locations may be caused by cross-talk between concurrent processes of parameter specification during response programming.