And then you can use both at the same time, which makes it even more confusing. In the simulator (never had to do it in the real world) things would go very exciting indeed in a very short amount of time if one advanced thrust levers from a derate 2 and high flex to full thrust... That demonstration usually resulted in not touching the thrust lever of the running engine on the next try, after all take off performance OEI is calculated on the used thrust level.