In a large bowl, add the all-purpose white flour, whole wheat flour and salt.
Knead in the shortening with your hands. Add warm water in small amounts, continuing to knead.
Once the dough is cohesive and elastic, gather it into a ball, and let it rest in the mixing bowl for at least 1 hour, covered with a kitchen towel. You can let the dough rest overnight in the refrigerator as well.
Divide the dough in to 24 equal pieces, and shape into round patties.
Heat a comal (griddle) on the stove. Press a divot into the center.
On a floured surface, roll out the tortilla until it is very thin.
Place the tortilla on the hot griddle for 30 seconds, then flip to cook on the other side for 2 minutes. Flip once more so that the tortilla balloons with steam.
Remove the tortilla from the comal and store in a kitchen towel until all the tortillas are rolled out and cooked.