The torque reaction on the props cancel each other out to prevent the boat from trying to run in circles.
A friend has a boat with two outboards. We were on a trip in it when the reverse rotation lower unit crapped out. He was unable to get a replacement reverse rotation lower unit, so put a standard rotation unit on it. Next time he took it out, he found he had gained 5 MPH in top speed, about a 10% increase. All the boat people said "That can't happen" but it did. He has no issues with the boat trying to run in circles.
