Virtual Reality (VR) and Augmented Reality (AR) technologies have been separately applied to enhance a wide range of design and manufacturing operations across various industries. Most VR applications are primarily focused on product prototyping and personnel training, whereas AR is commonly used to facilitate manual operations in manufacturing such as assembly and maintenance tasks. This study proposes a novel concept of real-time integration between VR and AR scenes to leverage their individual advantages. A prototyping system is implemented based on this concept to enhance workplace ergonomics in the overhead assembly of car bodies. An operator undergoes the assembly training at a virtual workstation within VR, while an ergonomics expert evaluates the operator's actions from a third-person viewpoint using AR. The expert can iteratively adjust the workstation setup, supported by automatic human posture recognition and biomechanical analysis, to reduce the risk of musculoskeletal injuries during the process. An exemplary case demonstrates the practical value of real-time collaborative applications between VR and AR in the context of human-centric smart manufacturing. A usability study is conducted to verify the prototyping system using both subjective and objective measures. Integrating various reality technologies serves as an effective approach to improving human well-being in the manufacturing environment.