Early Quenya
yullume
adverb. twice, *two times
hue yullume i hualqe
twice nine is eighteen
yú
adverb. twice
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
A word appearing as ᴱQ. yullume “twice” in the Early Qenya Grammar of the 1920s, a combination of ᴱQ. yúyo “two” and ᴱQ. -llume “times” (PE14/51, 84).
Neo-Quenya: In Tolkien’s later writings yúyo means “both” and atta means “two”. However, I would retain ᴺQ. yullumë “twice, two times” for purposes of Neo-Quenya since the prefix yu- is still used for double/twice in Tolkien’s later writings in words like yunquë “twelve” = “✱twice six” or yucainen “twenty” = “✱twice ten”.