Spray a casserole dish or 9x13 baking dish with cooking spray.
Arrange half the bread cubes in the dish, and top with cream cheese cubes. Sprinkle 1 cup blueberries and 1 cup strawberries over the cream cheese, and top with remaining bread cubes.
In a large bowl, mix the eggs, milk, vanilla extract, and syrup. Pour over the bread cubes. Cover, and refrigerate overnight.
Remove the bread cube mixture from the refrigerator about 30 minutes before baking.
Preheat the oven to 350°F.
Cover, and bake 30 minutes. Uncover, and continue baking 25 to 30 minutes, until center is firm and surface is lightly browned.
In a medium saucepan, mix the sugar, cornstarch, and water. Bring to a boil. Stirring constantly, cook 3 to 4 minutes. Mix in the remaining 1 cup blueberries.
Reduce heat, and simmer 10 minutes, until the blueberries burst. Stir in the butter, and pour over the baked French toast