The JV-90 was not designed to use both internal and V-EXP sounds at the same time. However, you can trick it into doing this. As an example, here are the settings for an internal bass sound split with a piano sound from the VE-JV1:
- Connect a MIDI cable from the JV-90 MIDI OUT to V-EXP MIDI IN and set the MIDI in switch to EXT.
- Press PERFORMANCE.
- Press EDIT.
- Press COMMON.
- Cursor to KEY MODE and set to ZONE.
- Press INT ZONE.
- Set Key Range for internal sound (e.g. part 1 lower C-1 and upper B3).
- Cursor down to LOCAL SWITCH and turn on the internal part (part 1 on, all others off).
- Press PART and select the internal sound (part 1, PRB-11; Woody Bass 1).
- Press TX ZONE.
- Set Key Range for external sound (part 2 lower C4 and upper G9).
- Cursor down to TRANSMIT PROGRAM CHANGE and set the sound (part 2, PRA-11, A. Piano).
- Cursor down to TRANSMIT SWITCH and turn on the external part (part 2 on, all others off).
Owner's Manual: https://www.roland.com/us/support/manual_archive/