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

双節棍[nunchaku] nunchaku (two linked fighting sticks)
一双[issou] pair (esp. of folding screens)
一卵性双生児[ichiranseisouseiji] identical twins
双紙[soushi] written work (esp. a bound text, as opposed to a scroll)
自己双対[jikosoutsui] self dual

双刃[moroha] double-edged
双暗車[souansha] twin propellers
双殻類[soukakurui] bivalves
双眼[sougan] both eyes

雨子[amago] land-locked variety of red-spotted masu trout (Oncorhynchus masou ishikawae)
御菓子[okashi] confections
お子さん[okosan] (someone else's) child
お子様[okosama] child (someone else's)
山梔子[kuchinashi] Cape jasmine (Gardenia jasminoides)

させ子[saseko] whore
梃子摺る[tekozuru] to have much trouble
螺子回し[nejimawashi] screwdriver
硝子[garasu] glass
獅子女[sufinkusu] sphinx