I personally would go nice and slow in getting more weight on them. i was in the same position not long ago, the breeder feeds the minimal necessary for survival and not much more. it took a couple months but they are both doing amazingly better, look healthier, exc without shocking their system. i would feed appropriately sized meals every 5 to 7 days (thats what we do, disclaimer, everyone is different) until you get up to a better weight... as long as they arent emaciated you should be ok. did you talk to the breeder about their weight and when they last ate so you know when to expect them to eat?