In a large skillet cook and stir beef, sausage and onion until browned; drain. Stir in taco seasoning and 3/4 cup water. Bring to boiling; reduce heat. Simmer, uncovered, about 5 minutes or until most of the water evaporates. Remove from skillet; set aside.
In the same skillet combine cheese and tomatoes. Cook and stir over low heat just until cheese melts. Return beef mixture to pan. (If mixture is too thick, stir in 1/4 cup milk.) Cook and stir mixture just until heated through. Serve warm with tortilla chips or corn chips.