First off Welcome to the forum!
Others on the board will have more experience than i with ferals but one of the first things that comes to mind are the pheromone products that soothe some (but definitely not all) cats. Feliway makes both a diffuser and a collar that calms some cats down.
Rescue Remedy has also proved useful for some.
A loud No! from you if and only if you ACTUALLY see the cat attacking the other can't hurt at all.
Hope others have additional ideas.