Matteo,
Select the voice effects section, select the initial harmony, then select echo on. Under the harmony type, select trill. Then you can set the trill rate of repeat. It functions when you press two or more keys.
On the PSR-3000 there is a Sweet Mandolin, which does this autmatically while pressing a single note.
Good Luck,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)