In a large saucepan bring the water to boiling. Stir in wild rice and brown rice. Return to boiling; reduce heat. Simmer, covered, for 40 to 45 minutes or until rice is tender and most of the liquid is absorbed. Remove from heat.
Meanwhile, in a 4-quart Dutch oven combine broth and garlic; bring to boiling. Stir in tomatoes, chicken, zucchini, and pepper. Return to boiling; reduce heat. Simmer, covered, for 5 minutes. Stir in cooked rice, thyme, and, if desired, Madeira; heat through.