I have a Mojo61 with the lower manual connected: can I play organ sound on the top keyboard and piano sound in the bottom keyboard?

Lower manual, when connected to Mojo61, will only works for tonewheel organ sound (VB3): it will be the lower manual of a double manual organ. With all the other sounds present in Mojo61, it will be silent. It's not possible to play two different sounds, for example piano in the lower manual and organ in the upper manual because Mojo61 is a monotrimbral instrument.