1st things 1st,are you in stereo mode on the receiver? If the display on the front says "pro logic" or anything other than stereo or 8-ch stereo,you may have it in a movie mode,which in case it will not be as loud as it can be.
The "A" & "B" are driven the same & when both are used the power is shared between the loads so,theoretically,it will not play any louder. But give it a try. Maybe you will like the results. Zone 2 is for speakers in another room & can be set up so that you can listen to one source of music on your main set up & another in,say,your office or bedroom.
I suggest reading the manual thoroughly.