According to the timeline of the shoftim in the Artscroll Tanakh (supposedly based on the Vilna Gaon), Barak and Devorah served as judges of Israel at the same time. Why was it necessary for them both to be shoftim?
The answer is in their story in Shoftim. Barak didn't want to be a Shofet unless Devorah agreed to lead with him.
Shoftim ch 4
8 But Barak said to her [Devorah], “If you will go with me, I will go; if not, I will not go.” 9 "Very well, I will go with you,” she answered....