Step 1Heat the oven to 325 degrees. Rinse the duck legs and pat them dry. Combine the cumin and the oregano and rub the mixture all over the legs. Season with salt and pepper to taste. Brush a shallow (13-by-9-inch) baking dish with the oil and arrange the legs in it in a single layer. Roast 1 hour.
Step 2Combine 2 chipotles and the beer and carefully pour over and around the legs. Roast 30 minutes longer, until the meat is very tender. Set aside until cool enough to handle, then shred the meat and skin with two forks (discard any skin that is too tough). Pour 1 or 2 tablespoons of the pan juices over the meat, just enough to moisten, and toss to mix. Keep warm.
Step 3Combine the tomatoes, cilantro and remaining chipotles (to taste) in a small bowl. Season with salt and pepper to taste. Combine the radishes and green onions in another bowl and set aside.
Step 4Heat the tortillas one at a time in a nonstick skillet or on a griddle until warm and pliable. Lay them out on a serving plate and top one side of each with a little cheese, then a little duck, then a little salsa and finally with the radish-scallion mixture. Fold over and serve at once, with the limes to squeeze over the filling.