Complex sentence: A camera with a special prism is used to create a matte simultaneously with the color footage, so that the footage can later be combined with another shot without the two images showing through each other.
Split into simpler sentences: A camera with a special prism is used to expose two separate film elements. This second film element is used to create a matte, so that the regular color footage can later be combined with another shot without the two images showing through each other.