You can steer a marathon canoe by changing sides, or with draw strokes and cross bow strokes. However, the most efficient way to steer the boat is to lean it and let it carve to one side or the other. Leaning the canoe allows you to maintain speed while turning.
In order to do this well, both paddlers need to keep the boat loose so that it can lean gently and effectively. The leans work the opposite of a bike; so lean away from where you want to go.The person in the back decides when to lean and how much, the person in the front stays loose to accommodate it.