Turn the remote keyboard switch to "off" in the 303. Then the xp-50 will only play on midi channels 1-7 and 10. Record xp-50 parts on 8,9, 11-16. and all other channels will be controlling the 303. Be sure to turn the rx switches off in the performance mode if you don't want the xp to play parts 1-7 and 10. (which are 303 parts.) This is all on my experience using an xp-80 w/ a mc-505. This may be a little different for you.
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"Blasphemy is a victimless crime."