Ditto what Joel said... with the caveat that the position ("critical distance") and height of the KM140 pair will be the main issue. Too close and low, the image will be too wide and will emphasize singers closer to the mic array. Too high and too far back will narrow the image and will pick up too much room. There are papers about critical distance to read, but if this is a non-critical learning experience, I'd start with the mic 3'/1m taller than the conductor, and positioned as equidistant as possible from the arc of singers. The size (width) of the group will largely determine that distance.
Good luck! Post a clip or two...