@Bizzymammabee if its both merrick before grain then their should not be a problem interchanging the food but with bullies and their sensitive tummies its hard to tell how they will react. now your bully could have a sensitivity to chicken. sarah does perfect with beef but give her to much chicken and she gets soft poo. no other symptoms just diarrhea from it. maybe add some rice for a few days and see if it clears up if not chicken might not be for her. keep us updated.