turn two

beast with two backs

Two people united in sexual intercourse in the missionary position.

Buggins's turn

A humorous, disparaging term for the appointment of a person by rotation or promotion, on the basis of length of service (it being that person's "turn") rather than merit or level of qualification.

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.

good turn

A good deed; a thoughtful or selfless act.

greater of two evils

The most harmful or unpleasant of two undesirable alternatives.