The discussion on why there are not two officers in each car made me think of a comment I wanted to make earlier. With only one officer per car, there might be implied pressure on the individual officer to "handle" things himself without calling for backup. Also, could be a "macho" problem if the officer was always calling for help.
I can not recall seeing 2 officers in one patrol car, I have seen two (2) detectives in one unmarked car. I know of a neighborhood that a patrol car will only enter with backup, but I think (2) two officers in one patrol car is saved for the movies. jmo idk