FB-
That's a great point about twins being gearbox torque limited under most operating conditions (AEO). While engine performance will typically be adversely affected by lower pressure and/or higher temperature atmospheric conditions, the performance of the gearbox remains largely unaffected.
Consider the
example of the EC145 twin. Each of its Arriel 1E2 engines has a T-O rating of 738shp and an MCP rating of 692shp at SL/ISA. The MRGB has a (AEO) T-O rating of 1040shp and an MCP rating of 848shp. You can see that the engines have plenty of power margin above the MRGB limit at SL/ISA, 42% at T-O and 63% at MCP. I don't have specific performance data for the engine, but it would require a reduction in power of 218shp per engine at T-O and 268shp per engine at MCP before matching the MRGB power limits (AEO).