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

列記[rekki] list
一列[ichiretsu] (a) row
下り列車[kudariressha] trains going away from the capital
逆行列[gyakugyouretsu] inverse matrix
区間列車[kukanressha] local train

系列[keiretsu] series
系列化[keiretsuka] putting in order
後列[kouretsu] rear
行列[gyouretsu] line
行列式[gyouretsushiki] determinant