If you have both a first and last name seals they will usually be opposites in that one will be yin (dark background) and one will be yang (light background). When used together near a signature it is uncommon to have two yin or two yang seals together as this lacks balance (and is visually less interesting). In terms of light and dark it doesn’t matter which one goes on top when used together.
The calligraphy illustrated above is by Qi Baishi (1864-1957) with signature and two seals which he carved for himself. The top seal has his name “Baishi” in red characters on a white background (yang) while the lower seal is a phrase with white characters on a dark background (yin).