fluidsynth/doc/polymono/poly_mono_2.txt

# How to add a new basic channel among others actual basic channels ?
# -------------------------------------------------------------------
# Perhaps you have already set several groups of basics channels and
# you want add a new one without modifying actual groups.
# Assuming following actual groups:

# Basic channel:  5, poly omni off(mode 2), nbr:  1
# Basic channel: 10, mono omni off(mode 3), nbr:  1

# Now we want to add  a new group 3: 
# Group 3 should have the following settings:
# basic channel 13, mode mono, omni off, (mode 3) composed of 2 channels.

# Use command setbasicchannels using numbered mode 3:
setbasicchannels 13 3 2
# or using named mode:
# setbasicchannels 13 mono_omnioff 2

Use basicchannels command to verify your settings
basicchannels

# end