two
In addition to the idioms beginning with two, also see fall between the crack (two stools); for two cents; game that two can play; goody-two-shoes; in two shakes; it takes two; kill two birds with one stone; know all the answers (a thing or two); lesser of two evils; like as two peas in a pod; no two ways about it; of two minds; put two and two together; that makes two of us; thing or two; wear two hats.