The extension you are dialing must be covered to the coverage answer-group that has the two phones. This can be done with a x-ported station, virtual station or a hunt-group with no members and the correct coverage-path with c1 as point 1. If that is not what you want add bridged-apperance button's on the second phone so it shares the apperance buttons of the first phone. Calling the first will also ring on the second.