The FP-80 allows you to select 1 tone for your left hand and 1 tone for your right hand, divided at a chosen split point. This called Split play. You can adjust the individual volumes of each tones in split play, by using the following procedure:
1) While in Dual mode, hold down the [TONE] button, and press the [BACKING] left arrow button to make tone 1 louder, and the [KEYBOARD] button to make tone 2 louder.*
* If the screen says "9-9", that means each tone is the same volume.