But N.1 engine overspeed, and its torque becomes 60%, wouldn't the other unaffected engine (N.2) lower torque to 20% to keep output XMSN at 80%?
All good advice above. Also note that a highside failure would never give 60% torque. In the failure case for N2 governing, the engine will probably go to the N1 topping value, and for N1 governing failure you may get maximum possible fuel flow which will take the engine to goodness knows where! When the latter occurs on ground it can spin the aircraft on deck. I had an N2 highside failure on ground and it was relatively benign - I immediately heard the Nr rising and closed both throttles to the IDLE stop. That type of failure will allow throttle response, whereas an N1 governor failure will require the throttles to be fully closed.