beast with two backs

eat for two

To be pregnant

fall between two stools

To fit into neither of two categories and, hence, be neglected or fail.

feed two birds with one scone

Synonym of kill two birds with one stone.

for two pins

Readily, with only the slightest encouragement.

greater of two evils

The most harmful or unpleasant of two undesirable alternatives.

hit two targets with one arrow

Synonym of kill two birds with one stone.

it takes two to make a quarrel

It takes two or more people to cause a quarrel; one cannot blame an argument entirely on the other side.

it takes two to tango

Some things need the active cooperation of two parties; blame is to be laid on both parties in a conflict.