Make them turn 30 degrees to camera, with their shoulder towards the lens. This will make them seem less wide, and when they turn their faces to camera, it stretches the neck a little to get rid of the double chin. Make sure they don't wear patterned, striped, or bright clothing. If they're in dark clothing, with a dark background that helps. Just a hair of separation between subject and background works great. Orson Wells was once photographed in a black turtle neck, against a black background, and looked great. If your subjects are together, get the larger one to go behind the thinner one, and have her look over the thinner subjects shoulder.
David Buzzard