Staying neutral between two fighting friends isn’t cowardice if you tell both of them the exact same thing.

Neutrality goes wrong when it is really two private alliances. You agree with whoever is in the room, each of them leaves thinking you are on their side, and the fight gets worse with your help. Saying one consistent thing to both people is a different act. It costs you the comfort of being agreed with, which is why it is not cowardice.

Decide your sentence before the next conversation, something you would be fine with either of them repeating. Then say that, and only that, to both. If one pushes for more, say plainly that you are not going to discuss the other behind their back.