Ah yes I know all to well the fussy kitten syndrome 🙂
I would seek a second opinion especially if you’ve done everything advised by your vet … For the two of them to be out of sorts at the same time would indicate a parasite or a virus to be present..
Have you been changing their food a lot? Or have they been kept on the same brand? Sometimes even the smallest change in food or food brand can cause persistent diarrhea ..
I did have a case with my Nutini that she had diarrhea for over a month! But we knew the cause of this see she had a very close call after she was spayed and was prescribed some seriously strong antibiotics which not only killed the bad bacteria causing the infection but as expected also killed the good bacteria in her gut .. It’s was horrible and took FOREVER to clear up but it so eventually..
I’m wondering if it might be worth giving them a small amount of goats milk natural yogurt this would have good bacteria and help the colon bacteria to thrive just incase there is an imbalance there that would cause pretty bad diarrhea if they are lacking in good gut bacteria which is essential in producing healthy stool..