Method:

1. Place both chiles in a bowl and cover with 4 cups boiling water; let sit until softened, about 30 minutes. Remove chiles from water; discard stems and seeds. Transfer chiles to a blender along with soaking liquid, and puree until smooth, at least 30 seconds. Set chile puree aside. 

2. Heat bacon fat in a 6-qt. saucepan over medium-high heat. Add beef and pork, and cook, stirring occasionally, until lightly browned all over, about 12 minutes. Using a wooden spoon, push meat to the perimeter of the pan, and add garlic and onion to the center of the pan; cook, stirring, until soft, about 2 minutes. Add oregano, cumin, and flour, stir ingredients together, and cook until fragrant, about 2 minutes. Add reserved chile puree and stock, and bring to a boil; reduce heat to medium-low, and cook, covered partially and stirring occasionally, until meat is very tender and sauce is reduced slightly, about 1 hour. Season with salt and pepper and serve ladled into bowls.