द्वेस्तीयः

Operational rule vidhi

The affix तीय comes in the sense of 'completer thereof' after the word द्वि.

Word by word (padaccheda)

Left context द्वेः pañcamī · ekavacanaSubstitute / term तीयः prathamā · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

द्वेः तीयः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix तीय comes in the sense of 'completer thereof' after the word द्वि।
This debars डट् ॥ Thus द्वयोः पूरणो = द्वितीयः 'the second' -- that which completes the two.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.