The FP-80 allows you to play two tones layered on top of each other. This called Dual play. You can adjust the individual volumes of each tones in dual 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.