A suffix appearing in the phrase imbi Menel Cemenye mene Ráno tie “between Heaven and Earth goes the path of the Moon” from Eldarin Hands, Fingers and Numerals (HFN) written in the late 1960s (VT47/11). In notes from 1968 Tolkien described this suffix in detail:

-ye “and” add[ed] to second of pair (sg. pl. — dual). In Q. still normally used of pairs usually associated as Sun, Moon; Heaven, Earth; Land, Sea; fire, water; etc. ... This ye is probably related to yé “lo” — (now see!) = kena, tira. Also in Q. yē or yea “What is more” (VT47/31).

A similar non-suffixal form ye appeared (untranslated) in the 1969 phrase Orome Valaron minya omenne ye Eldar “Orome first of (the) Valar met the Elves” (PE23/134). Christopher Gilson suggested that in this phrase ye might mean “*with”, which seems plausible to me.

Neo-Quenya: For purposes of Neo-Quenya, I would only use -ye to mean “and” with natural pairs like Anar Isilye “Sun and Moon”, an alternative to the dual conjunction yo. For “[together] with” I would use ó.