Bring milk and 2 cups water to a boil in a saucepan over medium heat, stirring occasionally. Gradually whisk in grits and salt; return to a boil. Cover, reduce heat to low, and simmer, stirring occasionally, 10 to 15 minutes or until thickened. Remove from heat; stir in cheeses until blended.
Layer brisket, grits, and Coleslaw in 12 (8-oz.) glasses; drizzle with barbecue sauce. Serve immediately.