I've been thinking, and I can't actually think of a circumstance where you'd actually need to play two at once live - because for recording purposes, you'd just record one track, then duplicate it - which I do often. I play live lots, but wouldn't want to use cubase for this - and even then I can't really see the point in layering sounds this way. We've been able to do it with MIDI instruments for years and it's always been a real faff, going from programme to programme, setting zones and stuff. I've always just used multiple keyboards!