yo conj. "and", "often used between _two _items (of any part of speech) that were by nature or custom clearly associated, like the names of spouses (Manwë yo Varda), or "sword and sheath" (*macil yo vainë*), "bow and arrows" (quinga yo pilindi), or groups like "Elves and Men" (Eldar yo Fírimor but contrast eldain a fírimoin [dative forms] in FS, where Tolkien joins the words with a, seemingly simply a variant of the common conjunction ar). In one source, yo is apparently a preposition "with" (yo hildinyar* = "with my heirs", SD:56).
Qenya
yo
conjunction. and
ko
adverb. reflexive
me·mittanelme i·mbarenna yo sasse né túro
we got into the house and there was Túro
sinome nimaruva, yo hildinyar tenn’ ambar-metta
Here will I abide, and my heirs, unto the ending of the world
ye
conjunction. and
yu
conjunction. and
The usual word for “and” in the 1910s was ᴱQ. ya(n) (PE15/69, VT40/8). It was derived from the early root ᴱ√YA (or possibly ᴱ√(D)YṆTṆ) in the Qenya Lexicon of the 1910s (QL/104-105). In his later writings, the usual word for “and” was Q. ar, but something like ya(n) survived in the “dual” conjunction yo “both ... and”.