Bass88, here are my comments on your request.
I too have a two person group that uses backing tracks (no sound engineer) and originally started with a laptop running Ableton for live performances. I could adjust anything I wanted on the fly. I started looking for an iOS solution for two reasons: 1. I wanted to be more portable (this included all hardware pieces not just the laptop), 2. I was actually spending too much time tweaking the mix/settings instead of focusing on playing.
With the change in approach, I spend more time mixing/EQ the backing tracks during creation in Logic. Then all I have to do on the gig is balance the live musicians with the backing tracks (3 pieces) vs. a dozen separate tracks, and I have also normalized the levels between songs so there is very little change needed once the initial level/EQ is set. It is very difficult to a be performer and be a sound engineer at the same time. The less sound engineering and more performing I can do on a gig, the better. This approach has made my gigs more enjoyable and less stressful. I truly believe if you do not have a sound person, the fewer tracks the better.
I also have some flexibility built in. I can use the same tracks if I have to play in a solo situation by having a separate track that would cover anything I need if my playing partner is absent. When we both are there, that track is muted and for solo work it is not muted. (the other approach I have used is bouncing a solo and duo version of the same song and putting one on each track and muting the one I don't need for the gig.
The last comment is about stereo tracks live. In my experience, they are not that useful. It may be because of the size of venue/group I work with, but no one I work with has any serious stereo effects, or mixes in stereo live, where this is required. Many groups run in mono to run the house off one side of the amp and monitors off the other side.
Bottom line for me......Keep It Simple Stupid!
Great discussion as always.
Can't say enough about Peter and ST3!