http://forum.ableton.com/viewtopic.php? ... pc40+apc20
This is the bit that I was reading:
"Combination Mode is now active when multiple Akai APC40/20s are in use. This means that the topmost controller selected in your preferences will control tracks 1-8, the second controller selected will control tracks 9-16, and so on. When an APC40 and an APC20 are connected at the same time, the APC20's buttons will default to Track Selection, and its faders will default to volume."
So, the ordering of controller selection in the MIDI preferences determines which controller selesct tracks 1-8, 9-16, etc.
I'm looking at replacing my Launchpad with an APC-40 and would be interested in expanding the number of tracks I can contolr using multiple APC-20s. Someone really needs to take one for the team here, spend their cash, and tell us how a mulit-APC set-up works.
