I have sheared a LDS shear pin and seen one engine scream up while the other follows, then drops down to nothing in an attempt to control the NR through torque matching/load sharing... as well as a low side ECU failure (old A Model). The former results in a lockout control of the high side failure engine, the latter a lockout control of the low side engine. Steps 2-3-4 of the "%TRQ Split between Engines 1 and 2" in TM 1-1520-237-CL covers both a high and low side failure starting with retarding the high power engine and seeing if either A. the % TRQ on the low side engine increases, or if B. % RPM R decreases (!), to determine if it is a high side or low side failure.
High side failures are identified by the engine hitting the POU (-700) or ODV (-701) limiter, which makes a hard jolting motion through the aircraft like a rev-limiter on an automobile engine. You can fly like this for as long as you'd like, as the engine will prevent itself from causing harm so long as the POU/ODV is working properly. Low side failures (not at high power requirements) usually are identified by a "hey, look at that, the torques are splitting... interesting" comment by the pilot not on the controls. I would prefer the checklist clarify before just saying to retard the high side engine, as it may be the only engine providing any power. Identifying a typical high side failure is almost foolproof (bang bang bang bang!) and a low side failure during high power demands requires the same response as decreasing RPM-R - fly out of it before worrying about the root issue.
Mike