The union of sun and moon in a single design symbolizes the union between the two halves of heaven, between male and female, complementing each other.

This design features also other symbols like the twist (eternal union), the fish hook (prosperity), to help them fish their own island out of the sea, and the lizard (spiritual powers) which is a symbol of good luck.

If they get inked on different sides by two people, half on the side and half on the front of the body, when they are put near (by embracing or standing side by side), they complement each other, as they do in the following sample pic:

for the part that is not visible from one side or frontally for one of the two is the visible part for the other.