Why not make it a little like Pokémon, in which you have to exchange the pokémon exclusive to your version with a friend that has the other version in order to complete de PokéDex?
There are two secret areas, one in a volcano (that requires a fire key), and another one inside an igloo (which requires an ice key).
At some point in the game, you get the chance to choose between the ice and the fire key; let´s imagine I choose the fire key.
I open the volcano area, making my fire key useless. Then I exchange it online whith a friend that chose and alredy used his ice key; then I can enter the igloo area, as well as my friend can now enter the volcano area.
Then everybody is happy!