In an ideal world, patrol cars would have two officers in them.  However, in this world of limited budgets for police forces, it is often the case that one-officer patrols are inevitable.

In terms of policing, two-officer patrols are superior.  If there are two officers in a car, they can see many more things than if there is only one officer.  A single officer has to concentrate on driving and will therefore miss many things.  With another officer in the car, this is less of a problem.  If there are two officers in the car, there is greater safety for the officers and more ability to apprehend suspects.  If a single officer pulls up to a crime scene, more suspects can escape than if there are two.  Two officers can also back one another up when entering a dangerous situation.

However, in the budget situation that most cities face, two-officer patrols are much harder to implement.  This means that police departments end up with one-officer patrols.  In this environment, the good point of one-officer patrols is that they allow a department with limited funds to have at least some police presence in more parts of a city at more times.