अनुवादे चरणानाम्

A द्वन्द compound or words signifying persons belonging to the different Vedic-schools, when the sense is that of repetition is singular.
The word चरण means a school or branch of any of the Vedas designated by the name of the person who founded such a branch and hence means such a person. The word अनुवाद means repetition by way of explanation, illustration or corroboration; that is to say, when a speaker demonstrates for some special purposes, a proposition which had already been demonstrated before, that is called anuvada.

This aggregate is used with the aorist of the roots स्था and इङ् only; as उदगात् कठकालापम् 'katha and kalapa arose (again, further demonstrating their doctrines)'; प्रत्यष्ठात् कठकौथुमम् 'katha and kauthuma established again.

Why do we say 'when meaning repetition'? Observe उदगुः कठकालापाः or प्रत्यष्ठुः कठकौथुमाः, when demonstration for the first time is meant. is the aorist of स्था and इन् which takes such a Dvandva and not any other verb or any other tense of these verbs; as:- अनन्दिषुः कठकालापाः and उदयन्ति कठकालापाः

Note :- This sutra is translated thus by Mr. Iyangar, 'Aggregation alone is admissible between words denoting persons belonging to the different charanas of the Vedas, when they are used along with the aorist derived from the roots stha and in in the sense of recitation of the charanas as already learnt, as distinguished from learning them the first time.' Prof. Bohthlingk's translation is, 'The Dvandva Compound of the names of Vedic-Schools is singular, when such a school is repeatedly mentioned equally along with another.'