Ian, yes it has midi in/out ports and the accomp data can be sent out via them. So when I get around to doing it I will post a note here.
The Yammies have a pretty good and easy to operate MIDI control section so I don't think you would have any issues.
That would be a good idea too, hooking the Privia to the s910. For a start the speakers would be better

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The advantage of the Privia I guess is it is so narrow front to back it can be used JUST as a keybed for the upper keyboard without losing too much in the way of access to the top.
You know what I mean, when you have to go reaching for buttons etc!!
It's only 11" front to back, so yeah pretty shallow.
Dennis