I don't agree. Under EASA-NCO there is a phrase about navigation means, which says "availability has to be guaranteed" or something along those lines.
You're missing the point. EASA NCO is not the current law even though it will most likely be at some point. So for now national regulations apply. I was referring to the German national law which might be different from other legislations. The German law does not prescribe any form of map nor does it mandate redundancy nor declare paper to be superior to electronic devices.
Where would EASA NCO prefer paper over digital devices and where does it require two map sources? It only talks about navigational equipment and in case navigation cannot be performed using visual landmarks requires the pilot to be able to safely conduct the flight in case one item of the navigational equipment fails. Navigational equipment is linked to the flight plan so it can hardly apply to a map. In my understanding is says you shall have e.g. a VOR in addition to a GPS for e.g. VMC on top.