Unless the conditions (e.g., water quality, poor diet, etc.) persist that brought on the infection, then other fish should not get infected.
If the other fish doesn't show it, it may be safe to assume it is not infected. Was each fish in its own bag? If they were together in one bag, then I would suspect the other is or may be infected.
The Maracyn Two for Saltwater fish treatment is a 5-day treatment. It is easy to do. Just add it to the water in a hospital or QT and maintain the concentration of antibiotic when you perform any water change. You turn off all organic removing processes (skimmer, carbon, absorbing materials, etc.) for the time.