I don't have the Tyros2 here, but I believe it's the same as on the S900.
Go to MIC SETTING/VOCAL HARMONY...VOCAL HARMONY TYPE...EDIT...scroll down the parameter list till you see HARMONY1/2/3 volumes.
Make your adjustments with the VALUE buttons...save your edit in USER.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.