There's a definition of a "critical engine" if you take off in a multi-engined aircraft: it's the one that causes you the most problems if it stops for whatever reason.
There's another definition, which I always thought depended on whether the props/fans/whatever rotated in the same or contrary directions. If they are going round the same way, then you'll hit the limits of yaw control sooner with a failure on one side than the other (the side depending on whether you're clockwise/anticlockwise) due to torque reaction.
In a twin, you've effectively become a single with an offset propulsion unit - depending on which way it's going round, the secondary effects will add to or oppose the yaw generated by the other power plant failing, thus leading to one engine being more "critical" than the other in terms of yaw containment.
That's how I always understood it but am ready to be corrected...