Have a DGX 505. I gave to a friend and he is having an issue with maintaining the dual voice. I have looked throughout the manual and can not find how to save the dual voice. no problem finding it and setting it up, but just cant figure how to maintain it. He is using FLUTE (096) as main instrument, but wants to use (095) as a dual with it. Can you please assist in locking this down for me if you can. There is no definitive answer in the manual.