The expression 'joined at the hip' is a hyperbole used most commonly to describe two or more entities considered inseparable -- consider Tom Sawyer and Huck Finn or Microsoft Windows and Internet Explorer.
In the media, 'joined at the hip' has become a cliché used to denote even the faintest of connections. AOL and Time Warner (both before and after the merger), radio and record industries, and sexism and feminism have all been described as such.
This phrase is also occasionally used to describe conjoined twins (with varying degrees of political correctness). From the synopsis for Side Show, a musical by Bill Russell and Henry Krieger on the life of Violet and Daisy Hilton:
Told almost entirely in song, Side Show is a moving portrait of two women joined at the hip whose extraordinary bondage brings them fame but denies them love.