Seperate them NOW! At least by a few rooms. Maybe take the healthy one to a friends house! There is a chance your rat has Mycoplasmosis... I highly infectious and DEADLY infection of the upper respitory system. You will need to keep her VERY hyderated and clean her cage more then usual. take her to the vet for medication, because if it is a respitory infection it will kill her in no time without medication.
It is VERY common in rats. it can be caused my messy cages, or a huge number of things. One of the most likely causes of death in rats. I'd imagine at least 50% of rats have it. They will sneeze and wheeze. If either one of your rats is sneezing, then that is probebly an emergancy. If the vet does give you medication for your rat, then make sure you get it for the healthy one. They can have Myco for the longest time without symptoms.
So take at least the sick one, or BOTH, to a vet as soon as possile! I'd hate to hear that one of them died. Or both of them. Put a humidifyer in your room or something to help them breathe. Whatever you do don't let either of them see ANY other rats, even EACHOTHER, untill ALL symptoms have been completly gone for atleast 3 weeks! I'm sure all the otehr rattie excperts would reccomend that. You can get a vet referral at www.ratsauce.com and some info on Myco there.
