द्व्येकयोर्द्विवचनैकवचने

Operational rule vidhi

For referring to two, the द्विवचन forms are used, where as for referring to one, the एकवचन forms are used.

Word by word (padaccheda)

Right context द्वि- एकयोः saptamī · dvivacanaSubstitute / term - prathamā · dvivacana

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
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

द्वि-एकयोः -

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

Commentary

The dual and singular case-affixes are employed severally in the sense of duality and unity.
This is also clear. When duality is to be expressed, a dual case-affix should be employed, and in the case of unit, the singular case-affix. Thus ब्राह्मणौ पचतः the two Brahmins cook. ब्राह्मणः पचति the Brahmin cooks.

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