Occasionally, 2 coincident bodies cannot be combined, despite appearing to meet all the requirements for the operation to complete. The following is a workaround for this problem: 1. Delete the coincident face(s) on each body. For example, if you put your hands together with the palms touching, we would delete the surfaces represented by the palms. This leaves 2 open surface bodies with shared edges. 2. Use Repair > Stitch to merge the 2 bodies along the shared edge(s).