Shame on me, but I completely forgot that old silly neat trick that allowed me to click the red "dots" in every MIDI instrument slot in order to play multiple instruments in unison mode.
I've checked the prefs, but could not find the appropriate setting.
I am still using Ableton Live 5 (5.2.2) on a new iMac i5 with Snow Leopard.
T.i.a.!