On the iPad, you can connect external USB audio hardware with multiple separate audio outputs.
If you want to specifically route the channels (left/right) of the 5 audio players included in SongBook+ to certain audio outputs of the external hardware, you can do this in the MediaPlayer settings of SongBook.
This can be used, for example, to play a synchronous click track for the drummer that only he hears.
Tip for Acoustic Metronome:
For the acoustic metronome of SongBook+, unfortunately, a specific audio output cannot be assigned. The metronome will always be output via the first two channels of the external hardware. However, if you do not assign these first two channels to any of the audio players, you can still output the metronome separately from the audio players.