If I remove the creation from setup() the sketch doesn't work, it hangs. I think it is probably necessary to give the RC522 I2C address AFTER the I2C hub has been set up, otherwise only the hub is directly connected to the I2C bus and not the RS522.
If I remove the global creation the sketch complains that "mfrc522 was not declared in this scope" originating in the function ShowReaderDetails().
BTW, I don't "want" two instances, I just cannot get the sketch to work without creating two instances. I don't think this can be good programming and that is why I am asking for a better solution.