In chapter 12,
Very early on you will come across two dying guards. You need them to stay alive and not use them as rat bait. To get past the first guard throw some Odoris to the left and walk past.
The second guard, use exstinguis on the flame ahead. Throw odoris into the house, then relight the fire with ignifer to trap them.
Go through the door to get the trophy. Not sure this constitutes saving them, but hey.