Ok, i made several trials today, and this seems to be the right set-up for making both Biab and an external source playing through Mainstage, on different channels. I used an mp3 player as external source, but assume it works wiht a microphone or a guitar cable too.
1) On Midi Audio Setup: create an aggregate device and check only
"integrated input" (i don't remember how exactly it's named on english systems

) and
"soundflower (16 ch)".
2) On Mac preference panel: set
both the audio
output and input on the aggregate device
3) Open soundflowerbed and check the aggregate device on the soundflower 16ch sub-menu
4) On Mainstage: set the input on the aggregate device, and the output (finally) on the built-in output or your external sound card.
At this point you should have Binab outgoing from channels 3-4 of mainstage, and the external source (the mp3 player in my case) outgoing from channels 1-2.
I deserve a drink now
