There were a lot of scifi mass market paperback titles back in the day where unless you know the exact title and author its lost to time.
There was this one scifi book I've been trying to find forever but no records (going on several decades now).
Two main characters were astronauts that crash landed on a earth-like planet with civilization in the middle ages. One became a trader, the other became leader of a militant order, where they had special techniques/knowledge that let them do amazing physical feats like hang by their fingertips on the edge of cliffs for several days.
