The chief pilot of the flight school that operated the multi-engine airplane reported that the pilot receiving instruction was taxiing the airplane to the runway at the non-tower-controlled airport and was "heads-in" looking down and researching information on his iPad when he heard someone yell, "stop". He quickly applied the brakes, but the airplane collided with an airplane stopped on the taxiway, holding short of the runway. The chief pilot further reported that the flight instructor was imputing radio frequencies and was unaware that the pilot was also looking inside the airplane. The instructor looked up just in time to hear the backseat passenger yell "stop" and see the collision. The safety coordinator of the flight school that operated the stopped airplane reported that, while holding short of the runway and performing the before takeoff checklist, the flight instructor and pilot-receiving-instruction felt a hard impact from the rear. The stopped airplane sustained substantial damage to the elevator. The safety coordinator and chief pilot both reported that there were no preaccident mechanical failures or malfunctions with their respective airplanes that would have precluded normal operation.