"Complement each other" means that two things cooperate with each other and complement each other. "If two people unite, their profits will be cut off" comes from the Book of Changes, which means that as long as two people unite, they can exert great power. It is also about the mutual cooperation and complementarity between two people.
"Concentrate on each other and bring out the best in each other" comes from Biography of Boyi in Historical Records, which means that only when two people cooperate with each other can they give full play to their abilities and functions.