The flight instructor in the twin-engine airplane reported that, during the second simulated single-engine failure approach, the pilot under instruction reduced the left engine power setting to 12 inches of manifold pressure, which activated the landing gear auditory warning horn. On downwind, the student performed the prelanding checks and asked the instructor if he should deploy the landing gear. The instructor told the student to delay the action and deploy the landing gear during the final landing checks because the student was struggling to maintain altitude. Shortly after turning to final, the student performed the final landing checks and provided verbal cues of completion but did not include the landing gear check. The instructor stated that he was distracted and focused on corrections because the student was struggling to maintain altitude and the runway centerline during the final approach. The instructor did not verify that the gear was extended. During the landing flare, the instructor realized that the landing gear was not extended, and he immediately took the controls. He leveled the airplane and landed on the runway with the gear retracted. The airplane sustained substantial damage to the engines. The instructor reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.