I'd adjust the music stand so it covers all but the KEYS on the top kb.
(You may have to move the arm to the front leg to do this)
That way, the music can be right in front of your face, and you can still play both boards.
I'd also put the Yammy on bottom for a few reasons:
1) It's bigger
2) It has no speakers, so the kn can be heard better - even with the 9k pumped INTO it.
3) I'd use the controls and styles of the Yamaha as the master, and just augment with the 6500. (personal taste)