For engines pre SB41020:
When one engine is selected to "OEI TNG" the selected engine will operate at lower limits (MCP) and govern the power turbine / main rotor speed. The opposite engine will govern at a reduced speed of 92% N2. This will be what your instructor was referring to. Perhaps they don't have updated training material?
For post SB41020 software:
Both engines produce up to half of the OEI power (140% Q).
The OEI Training mode logic uses twin engine power to simulate an initial single engine transient to “maximum torque” and rotor droop. Then both engines are used but they are limited to a maximum total PI of 140%.
In order to simulate the transient following an engine failure the engine selected to OEI TNG will accelerate to a maximum of 110% to then reduce to approximately 70%, to while the inoperative engine will decelerate to a minimum of approximately 25% TQ then accelerate to around 70% TQ.
Hope that helps.