In CMD 15-07, key data, assign a CAR/SIE (call arrival/secondary incoming extension)key to a button. The CAR/SIE will be the number of the extension that you want appearing on the other phone.
EX: you want x-299 to appear on a button on station 200.
Go to 15-07, bring up station 200, then place CAR/SIE 299 on a line key.