Cook the ground beef and onion in a 10-inch skillet over medium-high heat until the meat is well browned and the onion is tender (about 6 minutes). Drain the fat from the beef. Stir in 1 cup salsa, corn, oregano, salt and cumin. Cook the beef mixture until heated through.
Lightly spray a 15x19-inch jelly roll pan with nonstick cooking spray and place a tortilla on the pan. Top the tortilla with 3/4 cup meat mixture and sprinkle with 1/4 cup cheese. Cover with another tortilla. Spread with about 1 cup refried beans and 1/4 cup sour cream. Repeat the layers with the meat and cheese and top with a tortilla. Spread 1/2 cup salsa over top and sprinkle with 1/4 cup cheese. Assemble the second stack in the same manner.
Cover the pan with aluminum foil. Bake for 20 to 25 minutes or until heated through and the cheese is melted.
Cut the stacks into wedges to serve and top with dollops of sour cream.