Diki
If you want to fly with your car buy a plane. If you want to play organ buy an organ. As far as I know Yamaha arrangers can split the keyboard into Style+LH, LH and RH...not too sophisticated isn't it? If you want to replicate that on the lower keyboard you won't even get to the heels of the organ's functionality you describe above.
I was only explaining you don't need to touch your controller to change sounds, you can use the registrations to change one and up to 16 voices in the lower manual. Regarding to part splits it would depend on the controller and probably would be fixed if you can't access the panel.
Yes, this is not flexible enough for the genuine organ player but those features are not included in arrangers probably because it's not convenient for their business other than negligence.

Victor