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

一塁[ichirui] first base
一塁手[ichiruishu] first baseman
一塁線[ichiruisen] first-base line
堅塁[kenrui] stronghold
三塁[sanrui] third base

三塁手[sanruishu] third baseman
三塁線[sanruisen] third-base line
三塁打[sanruida] three-base hit
出塁[shutsurui] on base (baseball)