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

医学技術士[igakugijutsushi] medical technician
医学士[igakushi] Bachelor of Medicine
一言居士[ichigenkoji] person who is ready to comment on every subject
一士[isshi] private first-class (JSDF)
隠士[inshi] hermit

宇宙飛行士[uchuuhikoushi] astronaut
運転士[untenshi] driver
栄養士[eiyoushi] nutritionist
下士[kashi] noncommissioned officer
下士官[kashikan] non-commissioned officer