Thats a good start - doing the copy function, but that won't make both phones have the same DN extension number or make both phones ring when you call one. You basically have 2 options. Create a hunt group and assign that group to both phones as a key (shows up at the top of the phone with any lines that are assigned) making that key ring, OR putting an Answer Key of the first extension on the second extension and making that key ring.
Answer keys are in: Terminal&Sets, <extn #> Line Access, Line assignment, scrolling down to Answer Keys.
Hunt groups are down in System Programming.
If you need to keep the DN of one of your phones because of advertizing, the answer key is your solution. If not, the hunt group might work better as it shows up the same on both phones - and its functionality is the same - making training easy, and perhaps less confusing than answer keys.