Results from my research........

1: The couples are either contrasting or mirroring eachother....
If they are mirroring they are showing wearing matching or the same outift. The result of this usually makes them appear close.
If they are contrasting they are usually wearing outfits that clash and the result makes the couple appear unhappy or disfunctional.

2: Models are usally simular looking, creating a twinning feel, or they are totally differnt in the size, shape and looks, adding to a sense of disfunction.

3: Depending on the look of the couple, the body language either shows them as being close and affectionate or distant and awkward. Though sometimes a couple may be styled to look together, but their body language will contrast this...

4: Androgyny is used alot in couple photography and proves successfull in showing the unity of a close couple.