Combine 3½ cups 2% reduced-fat milk; 1¼ cups fat-free, less-sodium chicken broth; and 1 teaspoon salt in large saucepan; bring to boil. Gradually add 1¼ cups instant polenta, stirring constantly with a whisk. Cook over medium heat for 15 minutes or until thick, stirring frequently. Remove from heat; stir in ¾ cup (3 ounces) grated fresh Parmigiano-Reggiano cheese and ¼ cup mascarpone cheese. Serve immediately.