You can of course allow a DN call to ring on the agents DN key even if the agent talks with a customer on key 0 - this is an unusual request, so lets see if I remember the programming of the phone:
LHK should be set, so the DN can be found by an incoming call. If DN on key 3 then LHK 3.
RNG_MODE should be RNG 2. Line 2 (the DN) will then ring even if line 1 (Key 0) is busy.
As I set - not something I personally would recommend.