I don't think there are different law requirements. The requirement is to display the correct plate for the correct driver. If you are displaying multiple plates then you are not adhering to the law (in my opinion). Being practical is not relevant (this is the law we're talking about after all! (joke)).
Even practically, you cannot believe that displaying multiple plates makes sense? How is a guard to know if there is a learner driver there (who must legally be accompanied) or an N plater, who does not, or a full licence holder, if you are displaying both an N and an L plate?
What I predict will happen is what I saw in Australia this year when I was there. My niece had a P plate (same as our N plate). These were on a suction hook stuck to the window. When I was driving, the plates were taken down, and when she was driving, the plates were hooked back on. There were severe fines for not displaying the plates, or displaying them incorrectly when I was driving (with a full licence).