If I remember right, Stereo A feeds that channel to both sides in mono mode and Stereo B the other channel. I am not 100% sure, it is now more than 25 years ago, that I tried one. But its easy to find out: Just unplug the input to your left or right channels, play music and see what happens. Either A or B should be dead, the other one should play in mono mode from both of your speakers.
Cheers and happy listening!
Cheers and happy listening!