Purchase a box that is slightly longer than usual. Cut holes in both ends of the box for the assistants' head and feet to penetrate.
Prior to the trick, have an assistant curl up in the box at the foot end. Wheel or carry the box into the room where the audience is waiting.
Open the box and have the second assistant step inside. The assistant should appear to lie on her back, but should actually curl up at the front of the box. Have the first assistant stick her feet outside one end of the box and the second assistant stick her head outside the other end.
Saw the assistant in half. Ideally, the box should be cut in half prior to the magic trick, and there should be covers for both halves so that the presumed assistant's entrails are not revealed to the audience.