Reducing stress is an important part of the treatment, because a major cause of upper respiratory infections like this is a Herpes virus that is brought out by stress. Try to identify any stressors in the household and do your best to eliminate them. You can also buy a cat pheromone product like Feliway at the pet store, which also helps to reduce stress.
If they are very congested, then you can do a steam bath at home. Below is a link to an article that explains this and has helpful info about cat upper respiratory disease.
http://www.wikihow.com/Help-Your-Cat-Breathe-EasierOutbreaks like this are common amongst cat colonies. Make sure you continue treatment past the resolution of clinical signs (coughing, sneezing, discharge, etc.) and give the medication as prescribed. With good care, these outbreaks clear-up within the population over a month or so. I know it's tough to look at them like this, but stick with it and they should get better eventually.
I hope that helps and all gets sorted soon.