Combine macaroni, milk, water, garlic salt, and salt in a Dutch oven on a stovetop over medium-high heat. Bring to a boil, stirring occasionally. Reduce heat to a simmer and cook, stirring frequently so noodles don't stick to the pan, until noodles are soft and most of the liquid has been absorbed, 7 to 10 minutes.

Remove pot from heat and stir in margarine and pepper until melted. Stir in Cheddar cheese and 1/4 cup of the Parmesan cheese until melted.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Top pasta with bread crumbs, remaining 1/4 cup of Parmesan cheese, and bacon bits.

Place Dutch oven under the broiler until topping is lightly browned and crispy, 1 to 3 minutes. Top with chives and serve immediately.