For simulating hands-on tasks, the ease of enabling two-handed interaction with virtual objects gives Mixed Reality (MR) an expected advantage over Virtual Reality (VR). A user study examined whether two-handed interaction is critical for simulating hands-on tasks in MR. The study explored the effect of one- and two-handed interaction on task performance in a MR assembly task. When presented with a MR system, most users chose to interact with two hands. This choice was not affected by a user's past VR experience or the quantity and complexity of the real objects with which users interacted. Although two-handed interaction did not yield a significant performance improvement, two hands allowed subjects to perform the virtual assembly task similarly to the real-world task. Subjects using only one hand performed the task fundamentally differently, showing that affording two-handed interaction is critical for training systems.