Spread one side of each slice of bread with mustard. Top 1 slice with Cheddar and Swiss cheese; cover with other slice of bread, mustard side down. Spread outside of sandwich with margarine.
Heat skillet or griddle over medium heat until hot. Place sandwich in skillet; cook 2 minutes or until golden brown. Turn sandwich; reduce heat to medium-low. Cover; cook an additional 2 to 4 minutes or until sandwich is golden brown and cheese is melted.