Arvin,
Do you mean only one will ring when a call comes in? If you do, you can check "Disable ring on this line" under the DN page for the device.
If you do mean the incoming call can only be answered on one of the phones, I don't understand how the other is an extension. Unless you just want calls from the 2nd phone to present the same DN as the 1st phone when you make a call from it. If this is what you are looking for, go to the below link. It's for setting up multiple appearances of the same DN on one phone, but you can do the same using 2 different phones.
"Configuring a Single Dialed Number on Multiple Lines"
http://www.cisco.com/warp/customer/788/AVVID/single_dn_muli_line_14889.html
Hope this helps!
