Here is some Material Information to get you started, to do this we setup two Material Functions, one that is your base no scene capture reflection and one that is just your scene capture.
Here is the Base No Scene Capture Reflection (the texture Sample is just a field of solid white, but this would be where you could texture your mirrors if they are dirty or tarnished):
Here is the Scene Capture Material Function (whatever dirt or tarnish would also have to be added to the Render Target):
