Imagine that there is a line that connects the two subjects in a scene.  As soon as you have a two shot, the line is in effect.  You do not cross that line. The person on the left of the shot must always stay on the left.  The person on the right must stay on the right.  So each subsequent shot needs to follow this setup.  If the people change positions, the viewer will get confused.  The only time you can break this rule is if you actually show the camera move across the 180 degree line.

The camera can be placed anywhere in the green area. If you go outside of that area, the viewer will start to get disoriented when watching the scene.  The green area represents 180 degrees for this particular scene.

Over the shoulder shot from the camera’s right side, Position B

The two shot from the camera’s

point of view, Position A

Over the shoulder shot from

the camera’s left side, Position C