[JohnM]

You need to test speakers individually to assess what their contribution is, otherwise you risk applying EQ (or changing speaker position or applying room treatment to address a reflection) that is not properly targetted - when you see a peak in the combined response of the speakers, which one needs correction? You also need to test with both speakers active, particularly for low frequencies as much bass content is mono and having both speakers running drives the room differently; some resonances which are excited by an individual speaker are not excited when both are active, so they can often be left untreated.