据维基，Sandwich printing, also known as negative sandwiching, is a photographic technique created by combing two negatives or slides into the film carrier of the enlarger and printing on one sheet of paper as one image. This technique is generally used to create a juxtaposition of image elements that would not occur naturally in the world. Sandwich printing works in both black and white and color images.
应该是底片叠加印刷。