As a kid I read a book about two boys who hate each other and are opposites to each other (lanky red-haired runner vs. stocky black-haired wrestler). They're transported to a parallel world where they have to undertake some sort of quest. Miscellaneous memory-fragments:
- I think there's a magical yin-yang amulet.
- There's a wolf-spirit or wolf-god. Possibly called Fenrir?
- What we call magic they call science; what we call poetry they call magic.
- They're summoned into the other world by a bald man in a robe who looks a little like Varys from GoT.
Does this sound familiar to anyone?