Descriptive compositional etymology. The cup is "two-handled", it is a regular possessive compound. It requires a formal change from δύ- to δέ- (or from δί- to δέ-, assuming the etymologist was conscious of the fact that in compounds "two" appears as δι-), and the loss of the initial vowel of the second member ὀπή. Kaibel prints δύο ὦπας, whereas Eustathius adds the correct form ὀπάς. In both cases, notice the etymology relies on an inflected form, namely the accusative plural ὀπάς or ὦπας, which provides the second syllable of the etymon. In the gloss, "holes" is the object of "two have", therefore it stands in the accusative, and this was enough to justify this etymology