that ever walked on two legs

beast with two backs

Two people united in sexual intercourse in the missionary position.

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.

fresh legs

A substitute who has yet to play in the match, and therefore has more energy than those already on the field.

greater of two evils

The most harmful or unpleasant of two undesirable alternatives.

have legs

To have endurance; to have prospects to exist or go on for a long time.