It is not explained. Perhaps the term Midianites
is used rather loosely here, and the two are kings of the Midianites and their Amalekite allies respectively.1 In any case, as kings they are likely of higher rank than the princes the Ephraimites have already killed (Judges 7:25), and Gideon does not consider the war over until they, too, are captured and executed.
5 So he said to the men of Succoth, “Please give loaves of bread to the people who follow me, for they are exhausted, and I am pursuing after Zebah and Zalmunna, the kings of Midian.”