A fundamental tactical concept when working with another person is that of Areas of Responsibility.
When you are under stress it becomes harder to deal with large areas of possible threats. It’s basically tough to maintain a wide scope of awareness at any time, and under stress Tunnel vision starts to set in. So when you are moving through a dangerous area it makes things easier if you divide up who is watching what.
Therefore, if you are working with a buddy, it is wise that the two (or more) of you share the load. If you are looking in one direction, he should be covering another. That way the chances of one of you picking up a threat that the other may have missed increases substantially.
Theories of the best way to do this abound…who watches front? Who watches which flank? Should someone be dedicated to watch six? Should someone just glance back occasionally?
My opinion? I don’t really care as along as everybody is on the same sheet of music.