The earliest appearance of this root was ᴹ√TOTO- “repeat” from Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/109). The root appeared as √TŌ/OTO in a discussion of prefixes for “back” from around 1959, where Tolkien specified its meaning as “back as an answer, or return by another agent to an action affecting him, as in answering, replying, avenging, requiting, repaying, rewarding”; Tolkien also considered the forms √UTU/TŪ (PE17/166). In this 1959 note Tolkien crossed √TŌ/OTO through and seems to have replaced it with √KHAN. Tolkien mentioned the root √OT in a discussion of numbers from the late 1960s, but only to specify that “there was no primitive base OT-” (VT47/16).
Primitive elvish
rē
noun. single item take out of a (long) series
-rē
suffix. -rē
at
root. two, double, bi-, di-; back, re-; across, over, lying from side to side, two, double, bi-, di-; back, re-, [ᴹ√] again, twice; [ᴱ√] dual; [√] across, over, lying from side to side
re(n)d
root. kinsman, *cousin
nōrē
noun. kindred, race
taurē
noun. forest
tō/oto
root. back
(a)tata
cardinal. two
atata
root. two, two; [ᴹ√] again, back
atta
cardinal. two
ndorē
noun. land
tata
masculine name. Two
-(a)rē
suffix. abstract noun
atatya
adjective. double
khan
root. back
lairē
noun. *summer
ndōro
noun. land
yenā
adjective. female
As the main root for “two”, √AT dates all the way back to the Qenya Lexicon of the 1910s where ᴱ√ATA appeared with the gloss “dual” (QL/33). At this very early stage, its derivatives had mainly to do with pairs such as ᴱQ. aqi “a brace, a couple of, both” and at- “bi-, twi-”, whereas the earliest Quenya word for “two” was ᴱQ. yúyo (PE14/49). Later on, Q. yúyo became “both” whereas “two” became Q. atta.
In The Etymologies of the 1930s this root appeared as ᴹ√AT(AT) (Ety/AT(AT)) with variants ᴹ√ATTA which became the basis for Quenya atta “two” (Ety/ATTA) and ᴹ√TATA which became the basis for Noldorin tâd “two” (Ety/TATA); these numerals retained this form thereafter into the Quenya and Sindarin of the 1950s and 60s (VT42/24-26). In the 1930s, the ᴹ√TATA variant took on the sense “double” in Quenya, and the ᴹ√ATTA variant took on the sense “across” in Noldorin. The base root ᴹ√AT had the sense “again, back”, as seen in both Quenya and Noldorin prefixes ᴹQ. at(a)-, N. ad- “back, again, re-” (Ety/AT(AT)).
This root continued to appear in the 1950s and 1960s, retaining its various meanings of “two” (VT42/27), “back, again” (PE17/148) and “across” (VT43/33). Tolkien explored the origin and development of this root at some length in his essays on Elvish numerals from the late 1960s, connecting it to √AT(AR) “father” via various Elvish finger names (VT48/19).