We investigate the synergy of the two hands for virtual object manipulation. We report results from an experiment which suggest that the two hands together provide sufficient perceptual cues to form a frame of reference which is independent of visual feedback. The same is not true for one hand moving in empty space. Our interpretation is that users may not have to constantly maintain visual attention when both hands can be involved in a manipulation.
Our results suggest that using two hands can provide more than just a time savings over one-handed manipulation. Two hands together provide the user with information which one hand alone cannot. Our results also suggest that using two hands can potentially impact performance at the cognitive level by changing how users think about a task. Since the user can potentially integrate subtasks controlled by each hand without an explicit cost to switch between subtasks, this encourages exploration of the task solution space. Finally, to illustrate why one might expect this to be true, we present a task analysis which helps to reason about the differences between one and two-handed interfaces.