Breeder had them on raw when they came to me. Theyve not once had solid stools. I've tried dry, wet and back to raw but nothing works.
Did they have solid stools while at the breeders? What precisely does not solid stools mean -- soft but still in a pile, liquidly? Is there blood or mucous in the stool. Does the stool smell bad?
Trying different foods, if you didn't introduce the foods slowly into the raw, can actually cause or make diarrhea worse. If you are going to experiment with foods, you have to introduce the foods in small amounts and over a long period of time to give their guts time to adjust.
The cooked chicken breast that John recommended above should be used short term only -- no longer than a week -- to give their stomachs and intestines time to settle down. I would not feed yogurt or any other dairy product. If they are allergic to lactose, you'll only succeed in adding to the problem. They should be kept confined in one room with their litter boxes and toys until the diarrhea is gone and they are using the litter box 100%. Are you using the same litter that the breeder used? If not, you should do so for the interim.
Ask your vet specifically what was tested for and talk to the vet about giardia. Giardia is difficult to diagnose because the parasites are shed intermittently but should have appeared in either the PCR or an ELISA. Have them tested again. Have a full blood PCR done.
The diarrhea and pooping outside the box means that something is amiss. They are either sick from something or perhaps allergic to something. Whatever is going on is affecting both kittens so my suspicions are a parasite of some sort.
You should be changing the litter and disinfecting the boxes daily until this resolved as they will just keep reinfecting themselves with whatever they have.
Put them back on the food the breeder was feeding and add a little cooked rice to each meal -- a teaspoon at the most -- to see if it helps firm things up. Warm the food before giving it to them and take it up and throw away what they don't eat within 15 minutes. I'm concerned about them constantly begging for food -- they are not getting sufficient nutrition if they are doing that and it can hinder their development.
Cats are most commonly allergic to chicken and beef so if Nature's Raw contains those meats, that might be the culprit and you'll have to try different proteins such as duck, rabbit, bison, venison. But most importantly, ask the breeder what was happening before they left home and get your vet working on finding a solution.
Please keep us updated on how your furbabies are doing.