Combine cheddar and Swiss cheeses in a bowl. Sprinkle with flour, toss lightly to coat. Heat wine, or beer, in a medium saucepan until simmering but not boiling, reduce heat to maintain temperature. Gradually add the cheese mixture, by small handfuls, stirring constantly after each addition until cheese melts (mixture will appear curdled at first but smooths our as more of the cheese mixture is added). Stir in salt and garlic powder. Transfer to a fondue pot and keep warm. Serve with bread cubes for dipping.