The Bengals are going to keep pounding Green-Ellis up the middle while they use Bernard as their change-of-pace back to get around the edge and catch the ball out of the backfield. While Bernard will get more carries than he would just as a third-down back, Green-Ellis is still going to be the feature back.
There are several teams around the league who use a dual back system and many of them have had a lot of success at moving the ball on the ground. This doesn't mean that Green-Ellis will always be the feature back either. Depending on Bernard's ability to carry the ball between the tackles, he could become the team's main running back over time.
While Green-Ellis and Barnard are going to be soaking up the majority of the team's carries, Bernard Scott could find himself fighting for a spot on the team. He has struggled to stay on the field due to durability issues and, unless he proves his special teams worth or unless someone ahead of him is injured, he could be looking for a new team come August.
There is plenty of time between now and then, though, and anything can happen.