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

一眼[ichigan] one eye
一眼レフ[ichiganrefu] single-lens reflex camera
一隻眼[issekigan] sharp eye
遠眼[engan] farsightedness
遠眼鏡[toomegane] spectacles for far-sightedness

凹眼[ougan] cavernous eyes
凹眼鏡[ougankyou] concave glasses
開眼[kaigan] enlightenment
眼下[ganka] under one's eyes
眼科[ganka] ophthalmology