Place tequila, soy sauce, garlic, lime juice, orange juice, olive oil, salt, black pepper, jalapeño pepper, shallot and cilantro in a freezer-proof zip-top bag. Add flank steak. Seal and marinate 6 hours to overnight in the refrigerator.
Preheat gas or charcoal grill to 400°F.
Remove meat and discard marinade. Place steaks on oiled grates and grill for approximately 4-6 minutes on one side or until moisture starts to pool on the top and beef releases easily from the grates with tongs. Flip once, grilling on the other side for 4-6 minutes or until internal temperature reaches 145°F with a meat thermometer inserted into the thickest part of the steak. Transfer steaks to a platter and let rest for 3 minutes before slicing against the grain, into ¼-inch thick strips.
Warm the tortillas for 30 seconds on each side in a dry skillet or on the grill.
Stack 2 tortillas, add beef, toppings and lime juice if desired.
Combine pico de gallo ingredients in mixing bowl and toss thoroughly.