I’d look into whether the Korg has a ‘MIDI mirror’ function, that duplicates all splits and layers but in response to just one MIDI input channel. It is often included to allow simple home pianos (that likely only have one MIDI channel across the keyboard) to control the arranger despite it being split and layered.

I don’t know what Korg would call this mode, but I know quite a lot of upper end arrangers have it. Sadly, not the BK-9, but it did make an appearance with the BK-7m…