Have you tried giving them fiber and probiotics? The fiber (a spoon of canned pumpkin, or some powdered slippery elm/marshmallow root/psyllium husk, etc) should help with the diarrhea and the probiotics will help with restoring good gut bacteria. During times of inflammation, good gut bacteria start to be overtaken by bacteria that you don't really want growing in there. On that same note, when they have diarrhea, is there any mucous or blood? This could mean that their GI tract is getting irritated from all the diarrhea and you probably would want antibiotics at that point. You said her blood looked good. Did they look at B cell and neutrophil counts? Those are early responders to infections. Some bacteria and viruses are very easily transmitted from one animal to another. Depending on the type, its possible that one of your cats got sick from food or something else and passed it onto the other via licking, hanging around with, using the same litter box as the other cat. Have you called their food companies to find out if any other incidents have been reported? Also, has your vet ruled out one of them having and obstruction that could be causing the diarrhea (though two cats having an obstruction at the same time is probably really far fetched). Regarding your question about antibiotic vs steriod, their aims are different. Antibiotics would be prescribed to target bacteria that may be causing the diarrhea and vomiting. Steroids would act as anti-inflammatory meds.