Hi varsity,
phew, that's strange. i know, a few people had similar issues with synths with integrated keybed. i'm not sure, what the reason was there. you might wan't to do a search on "sustain" and look what helped them.
Only thing i experienced with the SonicCell:
When i disconnect the midi-cable while a key is pressed, it will obviously sustain. The interesting point there is: even if i plug the the cable back in and press&release the same key, it will further sustain. that's even the case, when i change to a different patch/performance. at first i had the feeling, that this individual key will hang. only thing, that turned it off, was pressing the panic-button on my keyboard or turning the SC off and back on.
In your case, it might be, that your first keyboard has a fault, not sending note-off on those keys. when you plug in a second keyboard, the note is still on in the SC, although it already decayed, so it will sustain also on that second keyboard. attach your second keyboard and turn the SC off and back on.
If it still happens, there has to be another fault. i doubt, it's the SonicCell, as there are no mechanical parts inside, that could get damaged. maybe the midi-cable?
I hope that helps, or one of the other threads about sustaining notes,
Good Luck!
Alex
gear:
Yamaha P80 stagepiano
Art X-15 Floorboard
SonicCell