Roland RD-700GX Owner's Manual page 42

Performance
Pressing Two TONE SELECT buttons Simultaneously
42
* Keep the [NUM LOCK] button off when carrying out this operation.
Even without pressing the ZONE SWITCH buttons, you can layer two tones by pressing
two TONE SELECT buttons simultaneously.
For example, if you want to layer a piano sound with strings, together press both the
[PIANO] button and the [STRINGS] button.
The [UPPER 1] button and [UPPER 2] button's indicators automatically light up, and when
you begin playing the keyboard, the piano and strings sounds are layered together.
When this is done, the tone for the button that is pressed down first is assigned to UPPER
1, and the other tone is assigned to UPPER 2. The UP1 and UP2 Part names are both
highlighted in the screen, with both Parts being selected.
Once you have selected two TONE SELECT buttons, pressing either TONE SELECT button
then selects that tone as the [UPPER 1] button's tone. The [UPPER 2] button's indicator
goes out, and that tone stops playing.