त्यदादीनि सर्वैर्नित्यम्

The pronouns त्यद् etc. when spoken of along with any other noun, (pronoun other than त्यद् etc.) are always retained as एकशेष (to the exclusion of others).
The list of 'tyadadi' pronouns has been given under sutra 1.1.27. They are 12 words beginning with 'tyad' and ending with 'kim;' when these tyadadi pronouns are compounded with any other noun, (or a pronoun, which does not belong to their class), they prevail, the rest are dropped. The word 'sarva' is used in the aphorism to indicate 'universality'; i. e. what ever may be the word in composition with them, the 'tyad &c.' are retained to the exclusion of others. The word 'nitya' is used to indicate that this is an obligatory rule, and not optional, as were the last rules.

Thus सः (he) + देवदत्तः (Devadatta) = तौ (they two).

Vart:- When त्यदादि words are compounded among themselves, the word which is read last in the order given in the Ganapatha remains, the rest are dropped. Thus यः + सः = यौ; यः + कः = कौ ॥ Thus यद् is read after तद् (see 1.1.27). So यद् will be retained and not तद्. Similarly किम् is read after यद्, and therefore किम् is retained to the exclusion of यद्.