put two and two together

band together

to unite, to come together

bang some heads together

Attempt to get results from a previously ineffectual or argumentative group, typically by coercion or force.

beast with two backs

Two people united in sexual intercourse in the missionary position.

birds of a feather flock together

People of similar character, background or taste tend to congregate or associate with one another; it is easier to establish friendships with people that one has a lot in common with.

bring together

To cause people to do something together; to bring about togetherness.

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.