I'm sorry to hear about your young cats' chronic loose stools, which seem to be worsening.
It will be important to describe what sort of loose stools they have to try and localize the problem. Loose, frequent stools with mucous or bright red blood point more towards large bowel diarrhea or colitis, whereas just watery stools with no mucous point more toward small bowel disease.
Chronic diarrhea does cause changes in motility of the gut which can also lead to bacterial overgrowth in the gut. Probiotics such as Fortiflora or Benebac can help replace appropriate bacteria.
Because multiple cats are affected odds are this is something contagious like a parasite or atypical bacteria.
I know that they likely have had stool samples checked to make sure that parasites aren't part of the problem.
In these cases I recommend several fresh stool samples from both cats be submitted to the laboratory to look for more unusual parasites. In house veterinary clinic testing may not be comprehensive enough or have the capability to diagnose these.
Have any other diagnostic tests been checked?
Has a fecal culture been check for abnormal bacteria such as clostridia or salmonella?
I see that you have tried diet changes. A food allergy/sensitivity or inflammatory bowel disease are possible but unlikely in multiple cats. If that is the case though they may need a low residue, easy to digest food or a hypoallergenic food to be able to properly digest and absorb their food and not have loose stools. Even if this is parasite or bacteria in origin I highly recommend a trial of either Hills i/d or Purina Veterinary Diets EN as both will cause intestinal inflammation if present for a long period of time. No treats, or table food while they are on this prescription food trial.
In young cats like this though parasites and abnormal bacteria are probably the most likely cause. Because eggs and cysts are shed intermittently they may not show up on every sample. Your veterinarian needs to look for protozoal parasites (like giardia, coccidia, toxoplasmosis, or tritrichomonas) as well as the usual worms. So I highly recommend that a few fresh fecal samples from each cat be submitted to the lab rather than checking it in the clinic as these can be hard to find, and sometimes we need to check samples from several days in a row to find them. They can check for abnormal bacteria as well.
Here is a link to very thorough testing option for kittens with chronic diarrhea: https://www.idexx.com/corporate/news/press-releases/20090116pr.html
Viral or bacterial infections are also possible causes of diarrhea in cats as is a dietary sensitivity, although usually dietary allergies/sensitivities show up later in life as they take a while to develop.
If they are not improving or treating for parasites doesn't help discuss further diagnostics with your veterinarian such as testing for feline leukemia, feline immunodeficiency virus or other viruses.
Please let me know if you have any further questions.