On CUCM > User Management > End User, there's a parameter called "Primary Extension".
CUPC soft phone and desk phone will use the DN specified in this paramter to do phone control.
Sure you can use different DN on soft phone and desk phone. But from CUPC perspective, you'll lose one of them. (You can still dial by pressing the keys on the desk phone though. Just not from CUPC).
Michael
http://htluo.blogspot.com