what you would do is figure out how much each dog is supposed to get throughout the day and split it in half. feed half in the am and the other half in the pm. it's not likely that they would get chubby by free feeding UNLESS you just plopped a bowl of food down without measuring out the amount that each dog should get. and even if the bowl was empty after they ate, not filling it back up. i wouldn't feed the others the allergy food, if they do develop a food allergy it will be harder to find a food that they will do well with if you introduce stuff like duck, fish, venison ect at a young age.