I believe UTI is a virus, yes? Well, until they all get clean bills of health, I suppose it is being passed back and forth. I know at the shelter where I volunteer, we wash or hands thoroughly between cat petting, feeding etc. so that we don't spread germs/viruses. I would change Vets or at least consult with another Vet just to be sure you and your furry friends are heading in the right direction treatment wise.

Keep us posted.