I read a book as a kid about a mysterious/magical door with 5 locks. I believe the door is in the middle of nowhere. Two characters meet at the door, each with one key and knowing each other I believe. One is trying to pick the remaining locks, but getting nowhere. They then go on to search for the remaining keys, together or apart I'm not sure.
"As a kid" means I probably read it somewhere between 1988 and 1994.