Cut the butternut and acorn squash in half and scoop out the seeds. Drizzle with olive oil and lightly sprinkle with kosher salt and pepper. Place face down on the prepared baking sheets and bake for 45 to 50 minutes. Remove from oven and allow to cool so that you can handle it without getting burned. (If you have a convection oven, do not preheat. Put the prepared squash on the baking sheets and put into the cold oven. Bake at 400° F/200° C for 20 to 25 minutes.)
Scoop all of the squash out into a large bowl and mash with a potato masher.
Melt the unsalted butter in the pot over medium heat. Add leeks and garlic and sauté until tender, about 10 minutes. Add the broth, mashed squash, and herbs and bring to boil. Reduce heat, cover and simmer until the squash is very tender, about 20 minutes.
Pour about ⅓ of the soup into the blender and puree. Pour puree out into the large bowl and repeat two more times. Pour the pureed soup back into the pot. Stir in the cream and sugar and bring to a simmer. Season with salt and pepper.
CROUTONS: Preheat the broiler. Butter one side of each bread slice and place, buttered side up, on the baking sheet. Broil until golden, about 1 minute. Turn over the bread slices and sprinkle on the cheese, then add the chopped sage and thyme. Sprinkle with salt and pepper. Broil until the cheese melts, about 1 minute.
To serve, ladle soup into bowls and top with the croutons, or serve them on the side.