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

矢っ張し[yappashi] also
一矢[isshi] an arrow
逸れ矢[soreya] stray arrow
遠矢[tooya] long-distance arrow (archery)
弓矢[yumiya] bow and arrow

光陰矢の如し[kouinyanogotoshi] time flies like an arrow
二の矢[ninoya] second arrow
無理矢理[muriyari] forcibly
矢印[yajirushi] arrow (mark or symbol)