Cats can get the upper respiratory infection so easily and it spreads quickly. It comes on in times of stress, even though they are partially protected if they have their basic vaccines. If pilling is hard, ask the vet for an injection of Convina, its an off label use but can be effective for URI and is a one time antibiotic that stays in the system for 2 weeks. The typical antibiotic is 2 doses a day for a week and can come in liquid.

I have also tried steaming, have the cat hang out in the bathroom while you are in the shower and the warm moist air loosens up the congestion. Then if you quickly warm some wet food you can keep them eating and hydrated.

Just a couple of thoughts for if your kitties get sick again.
Hope that your kitties are doing well, nice to see other cat people out there, I would have many more if I had the space!