Toast chopped ancho and pasilla chiles in skillet, about 5 minutes. Soften in hot water, drain.
Blend softened chiles with salsa verde, orange juice, lime juice, sugar, cumin, garlic, oregano, salt, and pepper.
Pour sauce into slow cooker. Season beef with salt and pepper, add to cooker, and stir to coat.
Cook on high 5 to 6 hours or low 8 to 9 hours. Shred beef in sauce.
Serve in tortillas with queso fresco and cilantro.