OK here's my Mall Ninja Sniper Simultaneous Shot System (MNSSSS):
Actually I think it might work
Each shooter activates a pressure sensitive switch when he's got a clear shot, that illuminates a green light on a control panel; each shooter is poised to take the shot while he's indicating green. You then have a team leader that is in charge of giving the shoot order. When all shots indicators are green, he gives the order: "shoot". All shooters fire at the same time.
You can also periodically rotate the shooters on station so that they can maintain a ultra high level of concentration.