द्विवचनविभज्योपपदे तरबीयसुनौ

The affixes तरप् (तर) and ईयसुन् (ईयस्) come in the sense of 'surpassing' after a प्रातिपदिक (Nominal Stem) or a finite-verb when it expresses (comparison between) two things or when that which is added to it (उपपद) is to be distinguished from another.
The word द्विवचन does not mean 'dual', but means an expression for two. विभज्य is irregularly formed by यत्, and means 'that which is to be distinguished or differentiated'. This debars तमप् and इष्ठन् ॥ The rule of यथासंख्य 1.3.10 does not apply here. First to take an example of द्विवचन or when comparison is between two things : thus: द्वाविमावाढ्यौ, अयमनयोरतिशयेन = आढ्यतरः 'both are rich, but he is richer amongst the two'. Similarly सुकुमारतरः, पचतितराम्, जल्पतितराम् 5.4.11: or with इयसुन् : as, द्वाविमौ पटू, अयमनयोरतिशयेन पटुः = पटीयान् 'more skillful of the two', लघीयान् ॥ Secondly to take an example of विभज्यते चोपपदे i.e. when a word in construction is to be differentiated: thus माथुराः पाटलिपुत्रकेभ्य आढ्यतराः 'The men of Mathura are richer than those of Pataliputra'. Here there are more than two things, but as the words are expressed in the sentence, the comparative degree in plural number is used. Similarly दर्शनीयतराः, पटीयांसः, लघीयांसः ॥ Of course all these words must be plural, as they refer to more than two persons.