Seals are often used in the opposite corners of a work of Chinese art. This way when you look from one seal impression to the other your eye travels across the center of the design. In Chinese these are sometimes called “balancing seals.” They will usually be in different styles, with one yin (dark background) and one yang (light background. They may also vary visually in shape and size in order to add visual interest.
The calligraphy above comes from one of the pavilions in the Lan Su Garden in Portland, Oregon (USA).