Sentence:  Eteocles and Polyneices are enemy brothers, that is, they are alike in spite of the difference erected by being on opposite sides of the Theban battlements.1 By uniting them with the <WSD>dual number</WSD> in the description of their deaths, Sophocles has the elders speak of them as indistinguishable in their hatred, spear work, and deaths. 
Meaning of the word marked with <WSD> tags:  Grammatical number denoting a quantity of exactly two.