Bill, that's a great solution, as long as you want simple one sound per manual, but once you get into more sophisticated splits and layers (like we see in some of the better Wersi demos), you need the MAIN arranger to be able to map that on the secondary keyboard for you, and also chose whether the chord recognition is coming from the lower end of the UPR manual, or LWR one (or both, or neither, or a combination of LWR and pedals, etc., etc.).

You need registration changes from ONE place to be able to do all that, not different commands from the arranger, lower manual keyboard, and pedals. Integration is what makes the organ such a powerful tool. Lose that, and you are simply running a normal multi-keyboard rig, and one thats' main sound-source is NOT set up well for this purpose...