Combine first 5 ingredients in a small bowl, stirring well. Spread 1 1/2 tablespoons mayonnaise mixture over 1 side of each of 4 bread slices. Top each with 1 ounce roast beef, 1/2 ounce corned beef, and 2 tablespoons cheese. Top each sandwich with one-fourth of the kimchi and 1 bread slice.
Heat a large skillet over medium heat. Generously coat both sides of sandwiches with cooking spray. Arrange sandwiches in pan; cook 6 minutes or until lightly browned on both sides, turning once.