I just had a good talk with Steve Demming from Yamaha, and he pointed out to me that the extra keyboard input MIDI channel doesn't JUST mirror the upper, but can do ONE sound or/and Chord Triggering function independent of the Upper. Sort of what I knew but he clarified it and I thank him...

However, he seemed a bit puzzled why anyone would WANT splits and/or layers on that lower manual. I told him that, rather than thinking of the lower manual as an addition to the upper, for those of us wanting to use a 76 or 88 as the MAIN manual and having the 61 arranger itself as more of a 'solo' addition to the lower, being able to split and layer the lower manual JUST as extensively as the upper is quite important.

I'm not sure I made a convert, but MAYBE this idea might get kicked up to the engineers one day... who knows!
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An arranger is just a tool. What matters is what you build with it..!