Seperating them works much better. I breed rats for myself and others, and when I first started I was using the harlem method and realized it never worked out well. Just use your concrete tub rack as the breeder rack and use a birthing rack for the females. I keep 1.6 in the breeder tubs but will do 1.8 if Im running short on space. Its funny that when I started I was doing everything opposite of what I do now, such as breeding methods and feeding.