Ellie will take some of Maggie's extra ounces.
We have the opposite problem here. She eats all the time and won't gain. Actually, she fluctuates all the time. We go from 5 1/4 - 6 because they use different scales at the vet and Ellie is just weird like that.
I would suggest more exercise. You could also add green beans. What treats do you give? Sometimes those can be very high in fat. I don't really think low fat dog food is necessary because they should just eat less of the regular (maybe not in Maggie's case) but sometimes low fat foods can help them fell full. I was looking at the ingredients in this food and I"m wondering, are you sure you want to feed it at all? It has menadione in it. Here is a link about it below but I cannot verify this info:
http://www.dogfoodproject.com/index.php?page=menadione