In a large bowl combine the masa harina, warm water, and salt. Knead until dough forms. If the mixture is too soft add one tablespoon of masa harina and if the dough is too hard add one tablespoon of warm water until you get a clay-like consistency. Cover the bowl with a wet towel for 10 minutes.
With oiled hands form three-inch balls with the dough. Using your finger make an indent in each ball then fill the dough with 1 tablespoon of cheddar cheese and 1 tablespoon of mozzarella cheese. Close the indent (if there is any filing leaking cover with a piece of extra dough).
Pat the dough back and fourth in your hands until you create a disk like shape. Repeat with the remaining balls of masa harina. Heat a skillet to medium high heat with 1 teaspoon of olive oil. Cook the pupusas for 2 minutes on each side until golden brown.