Getting a decent portrait of one person is hard enough. Photographing two people increases the challenge. Not only do you have to hope that they both look good/happy/whatever when you snap the picture but there are technical considerations. The two biggest are lighting and focus. In the studio I generally use two light sources to light the faces, though you can tell from the highlights in the eyes that the main light is to my right. Since eyes need to be sharply in focus, I coach the subjects to try to have their eyes on the same plane relative to the camera. I also use a high f-stop (8 or 11), which helps the depth of field.
Canon 1DII 1/400s f/8.0 ISO200 45mm