Preheat the oven to 375 degrees .
Arrange the bacon on a slotted broiler pan or a rack placed inside a rimmed baking sheet. Bake until the bacon is crispy, about 17 minutes.
In a small bowl, mix the mustard and maple syrup.
In a shallow dish, whisk the eggs and milk; season with salt and pepper.
Build the sandwiches: Spread the maple mustard on the bread. Top 4 of the bread slices with 2 slices cheddar, a quarter of the turkey, 3 slices bacon, 1 tomato slice and 2 slices Swiss cheese. Top with the remaining bread slices.
On a griddle or in a large cast-iron skillet, melt the butter over medium heat.
Coat the sandwiches in the egg mixture. Add to the griddle and tent with foil. Grill until the cheeses melt and the bread is golden brown, 2 to 3 minutes per side, adding more butter if needed. Cut the sandwiches corner to corner.