From the manual:
"OUTPUT
The configures the outputs to be STEREO (default), MONO or DUAL MONO. The STEREO option outputs discrete Left and Right audio material when connected to a stereo monitor or PA system.
In MONO mode, the Left output is the summed mono mix of VoiceLive 2 including effects. The Right output is dry with only Tone applied to the MIC or LINE input signal. This is useful when a live sound engineer wants to control the balance of wet/dry mix at the mix position. In this case, set LEAD MUTE to ON.
In DUAL MONO mode, the effected vocal comes from the left output and effected guitar is produced from the right output. This allows you to have a sound tech control the level balance between wet vocal (vocal with harmonies and effects) and wet guitar."
TT