I think this norm would do poorly in practice, because people would seek out antagonists they unconsciously knew would be flawed, rather than those who actually scare them.

A much better idea, I think, is the following:

1. Find someone who appears intelligent to you but is very ideologically different.
2. Offer to read a book of their choice if they read a book of your choice.
3. Give them a book you think will challenge them.

I'd suggest, however, that you not give the other person someone who will constantly mock their position, because usually this will only further polarize them away from you. Exposing oneself to good contrary arguments, not ridicule, is the way for human beings to update.