Melt the butter in a medium saucepan over medium heat. Add the flour and whisk until smooth. Continue to whisk for 1 minute, making sure the mixture doesn’t brown. Gradually whisk in the milk and cook, whisking often, until it just barely bubbles along the sides of the pan, about 5 minutes. Remove the pan from the heat, add the American and Monterey Jack cheeses and stir until melted. Stir in the jalapeño. If the sauce is too thick, whisk in more milk, 1 tablespoon at a time. Add salt and pepper to taste and keep the cheese sauce warm.
Toss together the shrimp, 1 tablespoon of the oil, the seafood seasoning and garlic in a medium bowl. Heat the remaining 1 tablespoon oil in a large skillet over medium-high heat. Add the shrimp and cook, stirring occasionally, for 3 minutes. Add the crabmeat and cook, stirring occasionally, until the shrimp are pink and just cooked through and the crabmeat is warmed through, about 2 minutes more.
Spread a layer of tortilla chips on a serving platter. Top with some of the shrimp and crab mixture and drizzle with some cheese sauce. Repeat with another layer of chips, shrimp and crab mixture and cheese sauce, building your nachos as high as you like. Sprinkle with the green onions and cilantro and serve immediately.