I have experience working both two officer and single officer patrols, with the majority of my career spent as a single officer patrol.
My understanding is that there are pros and cons for both. Beginning with single officer patrols, officer safety is always the first concern. However, it has been my experience that single officer patrols make better decisions early on during incidents. That is, an officer alone tends to work towards de-escalating an incident and calming the scene. I do understand that not all incidents can be handled this way, and that immediate action is sometimes necessary when the first officer arrives. However, with trained call takers and dispatchers, the appropriate number of officers are usually dispatched and help is generally only minutes away.
There are many incidents with two or more officers present where tragic results suggest that the solution isn't always more officers. Training is perhaps as important or more important than the number of officers involved.
It has been suggested that two officer patrols bring a different psychology to the same incidents. Two officer patrols can tend to become more aggressive early on during an incident response, because each officer knows he has immediate help available. Additionally, over time a lead officer emerges between the two. That is, one officer generally takes the lead and the second officer takes a support role. One officer generally becomes the decision maker and the other officer generally defers. Over time, the second officer becomes a concern for the department. An unatural dependence develops in the second officer that inhibits his/her competence, skills, and abilities.
Additionally, peer pressure and cultural norming become more prevelant in two officer patrols. That can be positive or negative, depending on the dominant personality. When it is negative, we eventually read about it in the news. that isn't to say that the same thing can't happen in single officer patrols, but rather that it is excellerated in two officer patrols.
The pros for each are perhaps more telling. First, it is possible that two officer patrols are better at detecting crime, four eyes being better than two. The passenger officer may observe more as he/she is relieved of driving responsibility. Additionally, when performance driving is required, it is helpful to have a second pair of eyes and another person to handle radio traffic while the other drives. Two officer patrols have immediate back-up, as discussed above.
Single officer patrols, promote the following. Officers automatically work to develop and improve the skills and competencies of successful officers. Officers are less likely to make rash decisions. More area is covered with the same number of officers. Peer pressure is reduced and cultural assimilation is retarded somewhat. Officers become more self-reliant, independent, responsible, and capable. Negative consequences of peer pressure and organizational culture are somewhat mitigated. There is a better allocation of man-power, as calls that only require one officer don't automatically receive two officers, as occurs with every two officer patrol response.
I have only scratched the surface of this issue, and hope that others will consider what I have written and add to and refine the list of pros and cons for one officer vs two officer patrols. I invite your comments!