I'm going off memory here, so I'm not 100% accurate, but close enough to demonstrate a point.
One of the big insurance companies did a study on teenage drivers a few years ago. The study found that when a teenager was driving alone, he/she only had around a 10% chance of having an accident. When there were two teenagers in the vehicle, the odds more then doubled to something like 30%. When there were three teenagers, it was well over 50%, and four teenagers in a car together raised it to something like 70 or 80%.
It was pretty obvious that they get distracted and have dificulty focusing on what they are doing. I see this with our kids all the time. It's also why I feel they get into allot of the accidents that they have of ATV's. They forget what they are doing all the time. I've pesonally seen my kids do this on bicycles to the point they have run of the road, hit trees and rode into drainage ditches.
And yes, it's mutch more likely to happen when it's the two of them playing, compared to just one of them.
Eddie