If the yaw that you're using to decide if there's a failure event is instead caused by an increased power output on the *other* engine you will have the same symptoms
I have yet to fly a GA piston twin which had any serious chance of suffering from increased power on any engine - even more so during the take-off sequence.
need the pilot to check the gauges to ascertain which is the causative engine
The gauges on a dead or dying piston engine which is windmilling give very similar readings to the working engine. In the workload of a single-pilot asymmetric departure, with an aircraft that's barely staying in the air let alone managing to climb, misreading the gauges is a very high likelihood.
Unless you're in IMC, your eyes should be outside the aircraft. Gauges are a cross-check, but they should only confirm what you already know.