Fry bacon: Place cast-iron pan over medium heat. Cut bacon slices in half, and place 4-6 halves into cold pan to begin to render fat. Fry in batches until brown and crisp without burning. Remove to paper towel-lined plate. Reserve fat.
While bacon is frying, slice bread to half-inch thickness. Slice cheese, approximately 8 thin slices per sandwich (4 cheddar, 4 pepper jack).
Assemble sandwiches: Top 4 slices of bread each with 4 slices of cheese (2 of each type), then a layer of 3-4 pieces of crisp bacon. Add 4 more slices of cheese, then the last piece of bread to each sandwich. Spread thin layer of mayo on outside of top piece of bread.
Turn heat in cast-iron pan down to medium-low. Coat bottom with half of the fat reserved from frying bacon. Place two sandwiches in the pan, mayo side down. Spread mayo on top slice of bread.
Cook sandwiches, watching closely, until first side has is golden brown. Flip carefully with tongs. Cover pan for two minutes unless cheese has already melted. Cook second side until golden brown and remove to a cutting board.
Pour remaining fat into the pan and repeat with the second two sandwiches. Slice sandwiches and serve!