TWO MECHANICS WERE WORKING ON THE #2 AIR SLIDE. THEY HAD NOT DISENGAGED THE AIR VALVE OR SHUT THE MANUAL SLIDE GATE. WHEN THE LOADOUT OPERATOR RETURNED FROM A SAFETY CLASS & TOOK OVER LOADING TRUCKS, HE DID NOT KNOW THE MECHANICS WERE WORKING ONT HE AIR SLIDE. HE SWITCHED THE KNOB TO LOAD OUT OF #2.