Pietro,
Press [SHIFT] & [1/PFM COMMOM]. The display will read "Part Detune". Press [1/PFM COMMON] several times until "Split POint". Change the split using the UP/DOWN buttons, and after settings, press exit. If you want to save your patch/performance, press Write, use UP/DOWN & 1-8 to select a memory location and press Write again.
Mgomes