Both Shulato and Gai are Brahma's successors. Shulato is the "light" successor and Gai is the "dark" one. There had to be two, because there must always be a balance between light and dark. That's why Shulato and Gai had to be enemies.
However, neither light nor dark by itself can use the power of Brahma. Only a balanced force can use the power of creation. Therefore, Shulato and Gai must merge in order to defeat Shiva...