The usual Sindarin word for “two” derived from ✶tata, a variant of √ATATA that was the basis for Q. atta “two” (PE17/14; VT42/27; Ety/TATA). It appears with both a short vowel as tad and a long vowel as tâd. I think the long-vowel form is the most correct one, but the short-vowel form is used for prefixes or when the word is not emphasized in a phrase.
Conceptual Development: The Gnomish Lexicon of the 1910s had G. atha “two”, but this was deleted and revised to G. atha as the “pl. 3rd person pronoun” (GL/20) and no new word for “two” was given.
(eavesdrop) (i lathra, i lathrar), also lathrada (i lathrada, i lathradar)