Thanks for your comments.
I've basically given up because I don't really get much time to work out how to use this stuff. I'll just live with a single loop but:
This is precisely what I want to do:
Play guitar (and why not some other instrument? live through Apogee Gio and Mainstage 2 on my Macbook Pro.
Create a concert with three loopbacks.
Create the loopbacks at set level so each loopback can have certain sounds (eg. Set 1: Clean sound with various pedals and patches, Set 2: Distorted sound with various pedals and patches, Set 3: Some other effect on the sound with various pedals and patches)
Assign a switch on the Apogee Gio so that I can go from from one loopback to another.
Have the option to hear all the loopbacks either all at once or in some other combination I choose on the spot but by being able to it with the Apogee Gio
Have the option to stop any of the loopbacks when I want with the Apogee Gio
Have the option to clear any of the loopbacks when I want with the Apogee Gio
Have the option to fade out any of the loopbacks when I want with the Apogee Gio
Have the option for separate loopbacks to be of different lengths
I reckon this can be done but when I tried it at set level, the other loopbacks go silent. I also cannot work out how to assign the particular function of moving from one loopback to another nicely without using the mouse. I think this is because the only way seems to be switching to another set, but then the previous loopback goes silent.
I don't think this will all work at concert level because I can't find a function that switches between loopbacks that I can map to the Gio.