Place sausage and bacon in two large, deep skillets. Cook over medium-high heat until evenly brown. Drain, crumble and set aside.
Butter one side of six slices of the bread. Placed slices butter side up in the bottom of a 9x13 in. baking pan.
In a large bowl, stir together cooked sausage and bacon with onion and cheese. Place half of his mixture on top of buttered bread. Then add another six slices of bread (no butter). Add remaining meat/cheese mixture. Beat together milk and eggs; pour over top of casserole. Cover with foil and refrigerate overnight.
The next morning: preheat oven to 425 degrees.
Bake, covered, in preheated oven for 45 to 60 minutes.