After living with my parents since the summer, my last 2 remaining girls are set to move back in with me soon

My other girl Pangur looked very slim/lethargic the last time I saw her, so I want to try and bulk her up while restricting what my tubby girl eats. I'm interested in making up my own mix but have steered clear of rat rations for a while because the smaller bags I'd buy would be mostly dust. Would love some suggestions on where to begin, and if I should be feeding them separately until Ash loses some weight. How much is appropriate for 2 rats, and is there more I can do to help Ash shed a few pounds?