Nick:

You are in to spend endless time...my suggestions are:

In the XS you must go to the MIDI page and put the clock in external.... and depending on what arps and the midi channels those have, you must either enable then and disable the ones you do not want.
On the 910, I would suggest to make registrations to setup the style, tempo, etc...and for the RIGHT hand I would experiment what sound change trigger from the 910 to the XS....and then save those, it may be in the 910 a banjo while the XS may play a horn or something else, that's what you get when to keyboards have totally different sound/voice map.
Good luck.
Let us know if this suggestion worked for you.

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mdorantes