Introduction: People with transtibial limb loss frequently experience suboptimal gait outcomes. This is partly attributable to the absence of the biarticular gastrocnemius muscle, which plays a unique role in walking. Although a recent surge of biarticular prostheses aims to restore gastrocnemius function, the broad design space and lack of consensus on optimal hardware and control strategies present scientific and engineering challenges. Methods: This study introduces a robotic biarticular prosthesis emulator, comprising a uniarticular ankle-foot prosthesis and knee flexion exoskeleton, each actuated by a custom off-board system. Benchtop experiments were conducted to characterize the emulatorâ\euro™s mechatronic performance. Walking experiments with one transtibial amputee demonstrated the systemâ\euro™s capability for providing knee and ankle assistance. Results: The -3 dB bandwidths for the knee exoskeletonâ\euro™s torque and motor velocity controllers were measured at approximately 5 Hz and 100 Hz, respectively. A feedforward iterative learning controller reduced the root-mean-squared torque tracking error from 6.04 Nm to 0.99 Nm in hardware-in-the-loop experiments, an 84% improvement. User-preference-based tuning yielded a peak knee torque of approximately 20% of the estimated biological knee moment. Conclusions: This biarticular prosthesis emulator demonstrates significant potential as a versatile research platform that can offer valuable insights for the advancement of lower-limb assistive devices.