I've also seen dual pots used as L-pad pots used directly to the speaker terminals so that voltage to the speakers could be reduced while still maintaining a near constant impedance to the amp output stage. These tend to be of very low ohm values so just measuring the end point resistance of the pot stages should tell you if it's designed to reduce volume at the input of the amp or the output of the amp. And if it's a higher value input dual pot it will most likely have a 'audio taper' rather then a linear resistance curve.
Lefty