If one choses to ride two abreast communicate before there is a car and decide who is going to go forward/back whatever. It's not essential but it is nice. I usually plan for the faster rider to go forward and depending on how far away the car is the other rider can soft pedal or coast a little. Also, if the other rider does not have a mirror I put myself closer to the center of the road as I can see a car a long way off in my mirror and react early if needed.