In a medium bowl, beat 4 oz softened cream cheese and 3/4 cup mayonnaise together until smooth and well combined.
Stir in 1/2 tsp garlic powder, 1/8 tsp seasoned salt, 1 cup shredded cheddar cheese, and 1 cup shredded mozzarella cheese until evenly distributed.
Lay out the French bread slices (cut 1/2" thick). Spread the cheese mixture evenly over the cut side of half the slices, then top each with another slice to form sandwiches, pressing lightly to seal.
Heat a nonstick skillet or well‑seasoned cast‑iron pan over medium heat until hot. Place the assembled sandwiches in the dry skillet.
Cook until the bottom is golden brown, about 3–5 minutes, pressing gently with a spatula; flip and cook the other side until golden and the filling is melted, another 3–5 minutes. If the bread is browning too quickly before the cheese melts, reduce the heat and cover the pan for 1–2 minutes to help the cheese melt.
Transfer the sandwiches to a cutting board, let rest 1 minute, then slice and serve warm.