I have been told that as of a year or two ago Dana calls all of the plays.
I don't know how it works, but even if Dimel is calling ALL the plays, it seems possible that there would be political/personal pressure to call some passing plays for the passing game coordinator.
That seems as good of an explanation as any in terms of why we came out passing the ball last year against TCU in the second half--Dimel wanted to keep Miller happy by throwing some (passing) bones to him.