EK-50 L - Sustain pedal and endless note

[briviomattia]

Hi everybody,
i'm a new Korg's user. I bought an EK-50 L. When i use the sustain pedal I have this problem (I try to explain):
If I set Piano sound in UPPER 1 and Strings sound in UPPER 2 and press sustain pedal, the notes of Piano become long but the Strings notes keep always on untill I release the pedal.
Is possible to set the sustain pedal only for an UPPER and disable the pedal for other UPPER?
For me it's very important to keep sustain on piano layer and not for other layers.

Thank you and sorry for my poor english.