What does coaching actually mean in chess?
In relation to that, how is it possible for a chess player to coach two different players? Do the trainees share their work, or does the coach work on different areas of the game with each player, and in that way avoid any "moral conflicts"?
Two of the most notable examples would be:
-Peter Heine Nielsen, who was coaching Anand for ten years (Wiki), before switching to his main WCC rival Carlsen in 2013 (although he was also helping Carlsen before 2013);
-Vladimir Chuchelov, who coached both Fabiano Caruana and Anish Giri (I am not sure whether he even coached them simultaneously at some point).
On the other hand, Gelfand stated in 2012 that he refused an offer from Kasparov to help him in his match against Anand, since Kasparov was helping Anand in his WC match against Topalov in 2010.
I cannot imagine coaches deliberately trying to keep secret some of the work they have previously done (it seems too artificial). Also, revealing all the secrets of the player they previously worked with, does not seem to me as "morally sound". So, what is the deal then?