As for the title I have 2 mercenaries, both with the skill "Posizione di difesa" (Italian, I think it translate to defence posture).
Whe I use both mercenaries only one can use the defence posture and, looking the combat log, I see only one of them lowering the damage received using the skill.
Character:
Jago_Rubes
Mercenaries:
- Muriathrid
- Bandarlog