I'm not too up on Yamaha gear but if it's anything like Korg, you more than likely need to go into the internal settings for the voices, I'm assuming sequence here, but even the presets need may need to be adjusted for L & R output.
That, at least is how my Korgs work, but I'm sure some of the other members, particularly those with similar gear will be able to steer you in the right direction.
Good Luck in your endeavours....L
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...L