The other option is to set a ring group off the original phone, dial *81 (assuming default numbering scheme still valid) on the original extn, press tick to add extn, enter other extn number, tick, left arrow, tick.
Now when the first phone rings, the second will ring as well irrespective of whether you have a dss key. If you have a display phone it will say call for XXXXXXXX so you know it is the other extn's call.