Any of you that have been to an OTB tournament, such as the US Open know firsthand that there are typically Demo boards on site somewhere. Sometimes in the playing hall, often in the hallway right outside of the playing hall (on the way to the bathrooms).
They do not have Demo Board police handcuffing players for glancing at the boards. Just saying. It is allowed. It is the very same position they would see if they were to look at their board - it is no different. They cannot move the pieces around so it is not considered assistance. ERGO it is not considered cheating or a violation of fairplay.
People really do need to get past the notion that the terms assistance and cheating/advantage are synonymous.
A chess related “tool” can be of assistance and at the same time have absolutely nothing to do with cheating. A 2nd private room assisted Spassky to perhaps focus better, relax or some such. Korchnoi’s concerns were likely not over cheating.
The concerns are simple. A playing field is offered for matches. Players know in advance the atmosphere and agree to play with the materials provided. Anything outside of this is of assistance ( not cheating) and by today’s standards technically is against Fair Play rules . Note it’s said Fair Play and not cheating play. The objective is equality. The same conditions for both participants.
None of this is of concern for unrated games. People here want to use a 2nd board for training purposes. Don’t players when learning new games practice as such- and play unrated until they feel comfortable? Any assistance being used in the learning stage gets discarded when they are confident to begin rated play.