Two hormones acting in different directions on the same phenomenon helps give better control to achieve a steady internal state, homeostasis. This means that if the phenomenon such as blood sugar deviates in either direction (too high or too low) there is a signal to return it to normal (insulin if too high, glucagon if too low). Both act to provide negative feedback.
Work Step by Step
Review the last few chapters on homeostasis and negative feedback to see how having two signals gives better control and surer homeostasis.