Two cyclists and the time is taken when the second rider crosses the line.

If you are smart you take short turns at the front, either the front rider drifts out and the second rides up the inside (the change over can be verbal or an elbow out to indicate 'your turn') Second rider rides as close as they can to the front bike to draft and conserve energy for their turn. We swapped every 400m or so. When we hit a cross wind we rode '3/4 wheel' I was tucked in partly behind him.

This is a good theory, but gets stuffed up on hills, or when one rider can't match the pace of the other. We kind of agreed on the speed we hoped to maintain beforehand and adjusted as we went.

Some events have 3/4 team members. These pacelines require more skill and practice - bit like you see on the track at Olympics and other team pursuit events. Usually with a paceline someone is always swapping places. It's fun but you do need to concentrate. I have only practiced it, but good for the bike skills.

Some people rode side by side, but it kinds of defeats the purpose.

Hope that helps