I had Yamaha NP-30 as a midi controller for my PSR-3000; NP-30 had a usual midi output. Must say, it didn’t send properly the velocity, I mean, when I played very softly it still sounded louder than if I used keyboard on PSR or when I connected Yamaha MM6 as a midi controller.

Speaking about using two keyboards one above another I personally have a problem with that because if a keyboard is just a little higher or lower my comfortable level I can’t really use it to the fullest – playing something a little bit complicated starts to feel impossible. When I’m sitting at my comfort level (which some people consider too high) I feel more in control of my fingers and a keyboard (the same goes when I played an acoustic piano for 3-4 hours per night).

If the idea is just to be able to play some additional parts (like flute or saxophone) on top of the piano, I guess, even 49 keys controller will do (most of voices don’t really have a wide range).