Over the shoulder
Simulator
The near shoulder is the biggest thing in frame and the hardest to light: too dark and it is a hole, too bright and it steals the shot.
The shoulder is the problem
An over-the-shoulder gives you a large dark object in the near foreground, blocking the lamps and taking up a third of the frame. Lighting the shot is mostly deciding what that object should be worth.
The near shoulder is foreground, not furniture
It is the biggest thing in the picture. Left black it reads as a hole in the frame; lit too much it becomes the brightest object and the eye goes there instead of to the face being covered.
About two or three stops under
A useful starting point for the near shoulder: enough to see its edge and the texture of the coat, not enough to compete. Under four stops and it stops being a shape at all.
It also blocks your key
Swing the key round to the camera side and the shoulder shadows the far face. Watch the angle: the usable range is narrower than on a single, and the lamp usually has to go higher to clear it.
Separation matters more here
Two heads and a wall in one frame merge easily. A backlight on the far subject is what keeps them off the background, and it is doing more work than in almost any other setup.
The reverse has to match
Whatever ratio you settle on here has to be mirrored when the camera turns round, or the cut will not work. That is a separate page, and the harder half of the job.
Why it gets shot so often
It carries geography and eyeline in one image: who is where, who is looking at whom. It is worth the lighting trouble because it does so much narrative work for free.