一も二も無く[ichimonimonaku] unhesitatingly
一言二言[hitokotofutakoto] a word or two
一石二鳥[issekinichou] killing two birds with one stone
一二に及ばず[ichininioyobazu] right away
一姫二太郎[ichihimenitarou] It's good to have a girl first and then a boy

うり二つ[urifutatsu] as alike as two melons (two peas in a pod)
黄道十二宮[koudoujuunikyuu] twelve signs of the zodiac
二度[futatabi] again
最小二乗法[saishounijouhou] least-squares method
三十二分音符[sanjuunibuonpu] 32nd note

御構いなく[okamainaku] please don't fuss over me
サーボ機構[saabokikou] servomechanism
シフト演算機構[shifutoenzankikou] shift arithmetic unit
ヒグス機構[higusukikou] Higgs mechanism (physics)
遺構[ikou] remains

一向に構わない[ikkounikamawanai] doesn't care at all
一般命令演算機構[ippanmeireienzankikou] general instruction unit (computer)
機構[kikou] mechanism
気構え[kigamae] readiness
虚構[kyokou] fiction

一段[ichidan] more
一段動詞[ichidandoushi] ichidan verb (one of the two main conjugation classes of verbs in modern Japanese, usually ending in -eru or -iru)
一段落[ichidanraku] completing the first stage
階段[kaidan] stairs
格段[kakudan] special

計画段階[keikakudankai] planning stage
決済手段[kessaishudan] method of payment
減段[gentan] reduction (of crop size)
五段[godan] Japanese verb group
五段動詞[godandoushi] godan verb (one of the two main conjugation classes of verbs in modern Japanese, usually ending in -u, -ku, -su, -tsu, -gu, -bu, -mu, etc.)