Don't panic - serious diahorrea in kittens is 1. highly infectious 2. liquid or nearly so and 3. frequent. And after this amount of time, if it was anything like that the kitten would be obviously dehydrated and ill if not dead. Cats are not dogs, their noses are dry.

I think this is dietary and you will do best by getting them both onto cooked chicken for a few days and not moving off that until they are really, um, solid.

liz