They have called plays for many years. They hey have built a good reputation as play callers. They just haven't done it in the same season together for the Bills yet.
The point is that because of their reputations that they have already established, they could be the best play calling combination the Bills have ever put together in 1 specific season.
Generally the Bills in the past have been good at having 1 good coordinator and struggle on the other side of the ball. With these 2 guys as play callers, the Bills have the opportunity to have a top 10 offense and defense.
This is interesting to think about because of the potential it brings for the Bills being a quality football team on both sides of the ball.