Book idea—about the male and the female (in a way) being perfect balance but

A book about the male and the female (in a way) being perfect balance but more like the light in the dark—like the light brother and the dark brother. Not racially, but morally and racially. You know how there's the dark and the light and they can live perfectly within the same being—and it's best when it's done that way. That's why nature provides about half and half male and female within a species?—(whatever male and female really mean, there's half of one type and half the other—and maybe a reason for this)