When the word saha 'with', is joined to a word the latter takes the 3rd case, when the sense is that the word in the 3rd case is not the principal but the accompaniment of the principal thing.,
Ex. पुत्रेण सहागतः पिता 'the father has come with the son.'
The same will be the result with the synonyms of सह as, पुत्रेण सार्द्धं 'with the son'. So also when the word सह is understood, as Panini himself uses in 1.2.65 वृद्धो युना &c.
Why do we say 'when not the principal.' Observe, शिष्येण सहोपाध्यायस्य गौः.,