Fill a large bowl with ice and water and set aside. Line a baking sheet with aluminum foil and set aside
Fill a large stockpot threefourths full with water. Place it over high heat and bring to a boil. Add the lobsters. Cook for 8 to 10 minutes until they turn red. Transfer the lobsters to the ice bath to stop the cooking
When cool enough to handle, using a sharp knife, halve the lobsters lengthwise. Remove the meat and place it in a small bowl. Remove and discard the innards. Rinse the lobster shells and place on the prepared baking sheet. Set aside
Preheat the broiler, with a rack in the middle position
In a skillet over medium heat, melt the butter. Swirl the skillet to cover the bottom completely
When the skillet is hot and the foam has subsided, add the garlic and shallots. Cook for 30 seconds. Gradually add the flour and whisk to combine. Cook for about 2 minutes, whisking until the mixture is thick and pale yellow. Whisk in the wine, mustard, cream, salt, and pepper until smooth
Stir in the Gruyère and Parmesan cheeses until melted and thick. Stir in the lobster meat until just coated. Remove from the heat. Divide the mixture among the lobster shells. Sprinkle the top of each lobster with crushed crackers.
Place the lobsters under the broiler for 4 to 5 minutes until the tops are golden brown. Garnish with the parsley and scallion. Serve warm.