Qenya
o(na)llume
adverb. at or during exactly the same time
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
The correlatives ᴹQ. ollume or onallume “at or during exactly the same time” appeared in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/100, 110), a combination of ᴹQ. on- “same” and ᴹQ. -llume “time”.
Neo-Quenya: For purposes of Neo-Quenya I would update this to ᴺQ. imallumë “at the very same time” using im- “same”.