I think many manufacturers of arrangers are missing out on the opportunity to design a second manual to go underneath the primary arranger that is nothing but keys. A lot of arranger's have a mode where a dumb keyboard with one channel can be automatically mapped to all the splits on the primary keyboard. All that needs to be added is the ability to map additional splits to the lower manual.

Controller keyboards are very inexpensive, as low as $300 or less. A dumb controller keyboard with no controls at all should be even less. Who WOULDN'T add a lightweight second manual to their arranger it was that inexpensive?