Yes it is possible to use dual voice.
On the right side of panel you will see UPPER-PART ON/OFF (it's just above the ONE TOUCH SETTING buttons).
Both buttons should be lit for dual voice.
The PART SELECT buttons above allow you to choose the voices for UPPER Right 1 and Right 2
You can adjust relative volumes by pressing BALANCE...located to the immediate left of the screen.
You can store the settings in a registration memory.
Ian
PS...you can find the PSR-S900's manual here...
http://www.yamaha.co.jp/manual/english/index.php The info is on Page 30.
[This message has been edited by ianmcnll (edited 05-01-2008).]