Stir together the flour, salt, baking powder, and sugar in a large bowl. Add the butter and shortening and cut into the flour mixture using a pastry blender until the mixture forms coarse crumbs. While stirring with a fork, slowly add the warm evaporated milk until it forms a soft dough. If the dough is dry, add an additional tablespoon of milk. Knead the dough in the bowl for 1 minute then cover the bowl and let the dough rest for 30 minutes. Form the dough into balls about the size of a golf ball and place on a greased baking sheet. Flatten the dough balls and cover with plastic wrap. Let the dough rest for 30 minutes. Lightly flour a work surface. Heat a griddle to medium-high heat. Flatten one dough ball at a time on the floured work surface with a rolling pin. Place the tortilla on the hot griddle and cook for 1 minute, turning the tortilla constantly, until the tortilla just starts to brown a little. Remove from the griddle and set aside. Repeat the process with the remaining dough. Serve the tortillas warm or let cool to room temperature and store in an airtight container.