to sleep soundly, to sleep without a worry, to rest easy

(figuratively) to be between a rock and a hard place, to be between Scylla and Charybdis, to be between the devil and the deep blue sea, to be caught in the middle, to be caught in the crossfire

two heads are better than one

better safe than sorry; a belt-and-braces/belt-and-suspenders approach always wins the day

masculine plural of bête comme ses deux pieds

(informal) in two shakes, in a flash, in a jiffy, in nothing flat

(simile, informal) dumbfounded, gobsmacked

to kill two birds with one stone

Alternative form of bête comme ses pieds

two heads are better than one