Brush the bread slices and onion slices with the olive oil. Place the onions on a hot griddle. Cook over medium heat until light golden brown. Place 2 tomato slices on each of 2 slices of the bread. Top each with a slice of Gouda cheese. Place a grilled onion slice on each of 2 more slices of the bread and top each with a slice of Chipotle Cheddar cheese. Place each stack on a griddle. Cook over medium heat until the cheese begins to melt. Assemble the sandwiches by inverting the Chipotle Cheddar cheese stack on top of the Gouda cheese stack. Top each with the remaining bread slices. Place the sandwiches untoasted side down on a griddle. Cook for 3 minutes or until light brown.