Like Quenya, Sindarin once had a dual inflection, used for pairs of objects, but in Sindarin this dual has fallen out of use:

The S. duals of nouns or pronouns early became obsolete, except in written works. A case occurs in Orgalaðad “Day of the Two Trees”, but since these S. nouns were all derived from Quenya names of the 6-day week, brought from Valinor, it may be due to an attempt to imitate Q. duals, such as ciriat 2 ships. In any case -d was later lost, and so we have argonath “the group of (two) noble stones” instead of *argonad (1972 letter to Richard Jeffery, Let/427).

This Sindarin dual was derived from ✶ata the “numerative dual ending”. The other ancient (and more common) dual ending was ✶-ū, but since final vowels vanished in Sindarin this u-dual vanished completely and there are only remnants of it in fossilized forms like the dual of lheweg “ear”:

• *slasū > lhahu > lhau, spelled lhaw “ears (of one person)” (PE17/62, 77; Ety/LAS²).

There are several clear -ad duals attested in Sindarin, however:

• Orgaladhad “Day of Two Trees (galadh)” (LotR/1110).
• samarad “*two neighbors (sammar)” (VT48/20).
• nobad “thumb and index finger as a pair, *(lit.) pickers” (VT48/5).

It is conceivable that the -ad dual remained active into the First Age (when names like Orgaladhad were introduced) and only fell out of use by the Third Age.

Conceptual Development: Tolkien mentioned an archaic dual form in the Gnomish Grammar of the 1910s that used the dual suffix -wi:

Nouns only preserve in living forms singular and plural. Traces occur of the commonest old duals but these are now confined to such as: ... mabwi “(a pair of) hands” ... talwi “feet” ... hent [or] henwi “eyes” ... hethwi “brother and sister” ... hunt (old -nt ending) “the nose (originally nostrils)”. Dual -wi, -wint, -wid [nominative, genitive, dative]. Adjectives follow in singular but without mutation (GG/10-11).

Signs of both the -wi/-ui and the -(n)t dual suffixes can also be seen in declension charts appearing in Gnomish Lexicon Slips (addendums to the Gnomish Lexicon) written towards the end of the 1910s (PE13/117-118):

• engant dual of engan “spear”.
• ceralt dual of ceral “jar”.
• golui dual of gôl “gnome”.
• gui(w) dual of gweg “man”.

Some dual forms are mentioned in the Early Noldorin Grammar of the 1920s, following roughly the same patterns:

• badui dual of ᴱN. bad “way” (PE13/120).
• [deleted] *duilent dual of ᴱN. duilen “swallow” (PE13/120 note #4, given in mutated from nuilent).
• him dual of ᴱN. hen(n) “eye” (PE13/122, replacing deleted henu).

There is also a remnant of the -(n)t dual in The Etymologies of the 1930s: the dual hent for N. hên “eye” (EtyAC/KHEN-D-E).