Thanks for using JustAnswer.com, I'm Dr. Dan and I'll try to help you.
I'm sorry your rat is having a rough time. Very likely your rat has a mycoplasma infection. This is common in rats and is very very nasty. The only thing that will truly work to beat back this problem is prescription antibiotics from a vet so if you want to do the best for your rat the vet cannot be avoided. However you can try two things that will ease her breathing until she can get proper treatment.
The first is to set her in front of a humidifier/vaporizer or set her in the bathroom with a hot shower running and have her breathe steam in for 15 min 2x daily. The other thing you can do to boost her immune system is to try the supplement HEALx booster: http://www.harrisonsbirdfoods.com/prodinfo.asp?number=HEALX BOOSTER, however it is better if the money were spent on much more effective antibiotics and vet visit.
I hope this helps, best wishes.
With these respiratory issues yes it is possible that your rat could die, as some of these infections can move very fast. However most of the time an E.R. visit is not necessary just try to get to a regular open hours vet ASAP, and preferrably tomorrow unless your rat is obviously struggling or gasping for breath... not just breathing hard.
Yes this is all you can do with out access to enrofloxacin and doxycycline antibiotics. It is not typically contagious however MANY if not MOST rats can get infected with it (typically at birth) and can be affected by it. I have seen many rats in a collection have the disease but since the organism can't live for even 1 second outside a living cell then it is not possible to be contagious. If your other rat is wheezing, then try the supplement and if it worsens you need to take that rat in as well.