WOW what a great question. I did the same. We rescued a pit and then rescued another after about a year. The second was food aggressive (because of a nasty old dog that use to growl at her while eating at her 1st home–before she was rescued) We separated their bowls about 20feet apart, fed at the same time and scolded if they got too close to one another. 2months later– both dogs eat fine and even can have their bowls next to each other. I think it also helped that we have food in the bowls constantly. The oldest dog wont eat if not hungry which helps a little. Good luck with your beauties!
