That’s the whole fun part - come up with an “obvious” solution and the try to figure out the problems or risks it would cause.

For example, an obvious solution is that the switch can't be changed from "RUN" to "CUTOFF" when the throttle isn't at idle - this could be done with a mechanical detent because they're right next to each other. Simple!

But now you've introduced additional failure modes - throttle sticks wide open and the engine is vibrating and needs to be shut down - so maybe you make it that the shutdown switch can work for ONE engine at any throttle position, but if TWO get turned off, both throttles have to be off, but that introduces ...